diff --git a/CMakeLists.txt b/CMakeLists.txt
index 9516971b9..cfffbdccc 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -361,6 +361,7 @@ list(APPEND libopenmc_SOURCES
   src/track_output.cpp
   src/urr.cpp
   src/volume_calc.cpp
+  src/weight_windows.cpp
   src/wmp.cpp
   src/xml_interface.cpp
   src/xsdata.cpp)
diff --git a/docs/source/io_formats/settings.rst b/docs/source/io_formats/settings.rst
index 38fc2d79f..ead440ae9 100644
--- a/docs/source/io_formats/settings.rst
+++ b/docs/source/io_formats/settings.rst
@@ -620,9 +620,9 @@ variable and whose sub-elements/attributes are as follows:
 :type:
   The type of the distribution. Valid options are "uniform", "discrete",
   "tabular", "maxwell", "watt", and "mixture". The "uniform" option produces
-  variates sampled from a uniform distribution over a finite interval. The 
-  "discrete" option produces random variates that can assume a finite number 
-  of values (i.e., a distribution characterized by a probability mass function). 
+  variates sampled from a uniform distribution over a finite interval. The
+  "discrete" option produces random variates that can assume a finite number
+  of values (i.e., a distribution characterized by a probability mass function).
   The "tabular" option produces random variates sampled from a tabulated
   distribution where the density function is either a histogram or
   linearly-interpolated between tabulated points. The "watt" option produces
@@ -668,8 +668,8 @@ variable and whose sub-elements/attributes are as follows:
   For a "mixture" distribution, this element provides a distribution and its corresponding probability.
 
   :probability:
-    An attribute or ``pair`` that provides the probability of a univariate distribution within a "mixture" distribution. 
-      
+    An attribute or ``pair`` that provides the probability of a univariate distribution within a "mixture" distribution.
+
   :dist:
     This sub-element of a ``pair`` element provides information on the corresponding univariate distribution.
 
@@ -1007,3 +1007,55 @@ sub-elements/attributes:
      sample points within.
 
      *Default*: None
+
+----------------------------
+``<weight_windows>`` Element
+----------------------------
+
+The ``<weight_windows>`` element specifies all necessary parameters for
+mesh-based weight windows. This element has the following
+sub-elements/attributes:
+
+  :id:
+    A unique integer that is used to identify the weight windows
+
+  :mesh:
+    ID of a mesh that is to be used for weight windows
+
+    *Default*: None
+
+  :particle_type:
+    The particle that the weight windows will apply to (e.g., 'neutron')
+
+    *Default*: None
+
+  :energy_bins:
+    Monotonically increasing list of bounding energies in [eV] to be used for
+    weight windows
+
+    *Default*: None
+
+  :lower_ww_bounds:
+    Lower weight window bound for each (energy bin, mesh bin) combination.
+
+    *Default*: None
+
+  :upper_ww_bounds:
+    Upper weight window bound for each (energy bin, mesh bin) combination.
+
+    *Default*: None
+
+  :survival:
+    The ratio of survival weight and lower weight window bound.
+
+    *Default*: 3.0
+
+  :max_split:
+    Maximum allowable number of particles when splitting
+
+    *Default*: 10
+
+  :weight_cutoff:
+    Threshold below which particles will be terminated
+
+    *Default*: :math:`10^{-38}`
diff --git a/docs/source/methods/neutron_physics.rst b/docs/source/methods/neutron_physics.rst
index 774575b46..7c1490647 100644
--- a/docs/source/methods/neutron_physics.rst
+++ b/docs/source/methods/neutron_physics.rst
@@ -1627,6 +1627,8 @@ cross sections.
 Variance Reduction Techniques
 -----------------------------
 
+.. _survival_biasing:
+
 Survival Biasing
 ----------------
 
@@ -1677,6 +1679,47 @@ default, the cutoff weight in OpenMC is :math:`w_c = 0.25` and the survival
 weight is :math:`w_s = 1.0`. These parameters vary from one Monte Carlo code to
 another.
 
+Weight Windows
+--------------
+
+In fixed source problems, it can often be difficult to obtain sufficiently low
+variance on tallies in regions that are far from the source. The `weight window
+method <https://doi.org/10.13182/FST84-A23082>`_ was developed to increase the
+population of particles in important spatial regions and energy ranges by
+controlling particle weights. Each spatial region and particle energy range is
+assigned upper and lower weight bounds, :math:`w_u` and :math:`w_\ell`,
+respectively. When a particle is in a given spatial region / energy range, its
+weight, :math:`w`, is compared to the lower and upper bounds. If the weight of
+the particle is above the upper weight bound, the particle is split into
+:math:`N` particles, where
+
+.. math::
+    :label: ww-split
+
+    N = \min(N_{max}, \lceil w/w_u \rceil)
+
+and :math:`N_{max}` is a user-defined maximum number of splits. To ensure a
+fair game, each of the :math:`N` particles is assigned a weight :math:`w/N`. If
+the weight is below :math:`w_\ell`, it is Russian rouletted as described in
+:ref:`survival_biasing` with a survival weight :math:`w_s` that is set equal to
+
+.. math::
+    :label: ww-survival-weight
+
+    w_s = \min(N_{max} w, f_s w_l)
+
+where :math:`f_s` is a user-defined survival weight ratio greater than one.
+
+On top of the standard weight window method described above, OpenMC implements
+two additional checks intended to mitigate problems with long histories. First,
+particles with a weight that falls below some very small cutoff (defaults to
+:math:`10^{-38}`) are killed with no Russian rouletting. Additionally, the total
+number of splits experienced by a particle is tracked and if it reaches some
+maximum value, it is prohibited from splitting further.
+
+At present, OpenMC allows weight windows to be defined on all supported mesh
+types.
+
 .. only:: html
 
    .. rubric:: References
diff --git a/docs/source/pythonapi/base.rst b/docs/source/pythonapi/base.rst
index 2dea91bb0..ff849b92c 100644
--- a/docs/source/pythonapi/base.rst
+++ b/docs/source/pythonapi/base.rst
@@ -24,6 +24,7 @@ Simulation Settings
    openmc.Source
    openmc.SourceParticle
    openmc.VolumeCalculation
+   openmc.WeightWindows
    openmc.Settings
 
 The following function can be used for generating a source file:
diff --git a/include/openmc/constants.h b/include/openmc/constants.h
index 2596a0334..5a099f956 100644
--- a/include/openmc/constants.h
+++ b/include/openmc/constants.h
@@ -358,6 +358,10 @@ constexpr int CMFD_NOACCEL {-1};
 
 enum class GeometryType { CSG, DAG };
 
+//==============================================================================
+// Variance Reduction constants
+constexpr double DEFAULT_WEIGHT_CUTOFF {1.0e-38}; // default low weight cutoff
+
 } // namespace openmc
 
 #endif // OPENMC_CONSTANTS_H
diff --git a/include/openmc/particle_data.h b/include/openmc/particle_data.h
index 06fccc092..7d0382385 100644
--- a/include/openmc/particle_data.h
+++ b/include/openmc/particle_data.h
@@ -305,6 +305,9 @@ private:
 
   int n_event_ {0}; // number of events executed in this particle's history
 
+  // Weight window information
+  int n_split_ {0}; // Number of splits this particle has undergone
+
 // DagMC state variables
 #ifdef DAGMC
   moab::DagMC::RayHistory history_;
@@ -426,6 +429,9 @@ public:
   double& collision_distance() { return collision_distance_; }
   int& n_event() { return n_event_; }
 
+  int n_split() const { return n_split_; }
+  int& n_split() { return n_split_; }
+
 #ifdef DAGMC
   moab::DagMC::RayHistory& history() { return history_; }
   Direction& last_dir() { return last_dir_; }
diff --git a/include/openmc/physics.h b/include/openmc/physics.h
index b2e72e047..32a43e26d 100644
--- a/include/openmc/physics.h
+++ b/include/openmc/physics.h
@@ -89,6 +89,11 @@ void inelastic_scatter(const Nuclide& nuc, const Reaction& rx, Particle& p);
 
 void sample_secondary_photons(Particle& p, int i_nuclide);
 
+//！Split or Roulette particles based their weight and the lower weight window
+// bound.
+//! \param[in] p, particle to be split or rouletted with the weight window.
+void split_particle(Particle& p);
+
 } // namespace openmc
 
 #endif // OPENMC_PHYSICS_H
diff --git a/include/openmc/settings.h b/include/openmc/settings.h
index f14f6c15f..50be80a50 100644
--- a/include/openmc/settings.h
+++ b/include/openmc/settings.h
@@ -55,6 +55,7 @@ extern "C" bool trigger_on;        //!< tally triggers enabled?
 extern bool trigger_predict;       //!< predict batches for triggers?
 extern bool ufs_on;                //!< uniform fission site method on?
 extern bool urr_ptables_on;        //!< use unresolved resonance prob. tables?
+extern bool weight_windows_on;     //!< are weight windows are enabled?
 extern bool write_all_tracks;      //!< write track files for every particle?
 extern bool write_initial_source;  //!< write out initial source file?
 
@@ -99,6 +100,7 @@ extern std::unordered_set<int>
   statepoint_batch; //!< Batches when state should be written
 extern std::unordered_set<int>
   source_write_surf_id; //!< Surface ids where sources will be written
+extern int max_splits; //!< maximum number of particle splits for weight windows
 extern int64_t max_surface_particles; //!< maximum number of particles to be
                                       //!< banked on surfaces per process
 extern TemperatureMethod
diff --git a/include/openmc/weight_windows.h b/include/openmc/weight_windows.h
new file mode 100644
index 000000000..12113d528
--- /dev/null
+++ b/include/openmc/weight_windows.h
@@ -0,0 +1,99 @@
+#ifndef OPENMC_WEIGHT_WINDOWS_H
+#define OPENMC_WEIGHT_WINDOWS_H
+
+#include <cstdint>
+#include <unordered_map>
+
+#include <hdf5.h>
+#include <pugixml.hpp>
+
+#include "openmc/constants.h"
+#include "openmc/memory.h"
+#include "openmc/mesh.h"
+#include "openmc/particle.h"
+#include "openmc/vector.h"
+
+namespace openmc {
+
+//==============================================================================
+// Non-member functions
+//==============================================================================
+
+//! Apply weight windows to a particle
+//! \param[in] p  Particle to apply weight windows to
+void apply_weight_windows(Particle& p);
+
+//! Free memory associated with weight windows
+void free_memory_weight_windows();
+
+//==============================================================================
+// Global variables
+//==============================================================================
+
+class WeightWindows;
+
+namespace variance_reduction {
+
+extern std::unordered_map<int32_t, int32_t> ww_map;
+extern vector<unique_ptr<WeightWindows>> weight_windows;
+
+} // namespace variance_reduction
+
+//==============================================================================
+//! Individual weight window information
+//==============================================================================
+
+struct WeightWindow {
+  double lower_weight {-1}; // -1 indicates invalid state
+  double upper_weight {1};
+  double survival_weight {0.5};
+  double weight_cutoff {DEFAULT_WEIGHT_CUTOFF};
+  int max_split {1};
+
+  //! Whether the weight window is in a valid state
+  bool is_valid() const { return lower_weight >= 0.0; }
+};
+
+//==============================================================================
+//! Weight window settings
+//==============================================================================
+
+class WeightWindows {
+public:
+  // Constructors
+  WeightWindows();
+  WeightWindows(pugi::xml_node node);
+
+  // Methods
+
+  //! Set the weight window ID
+  void set_id(int32_t id = -1);
+
+  // NOTE: This is unused for now but may be used in the future
+  //! Write weight window settings to an HDF5 file
+  //! \param[in] group  HDF5 group to write to
+  void to_hdf5(hid_t group) const;
+
+  //! Retrieve the weight window for a particle
+  //! \param[in] p  Particle to get weight window for
+  WeightWindow get_weight_window(const Particle& p) const;
+
+  // Accessors
+  int32_t id() const { return id_; }
+  const Mesh& mesh() const { return *model::meshes[mesh_idx_]; }
+
+private:
+  // Data members
+  int32_t id_;                     //!< Unique ID
+  ParticleType particle_type_;     //!< Particle type to apply weight windows to
+  vector<double> energy_bins_;     //!< Energy bins [eV]
+  vector<double> lower_ww_;        //!< Lower weight window bounds
+  vector<double> upper_ww_;        //!< Upper weight window bounds
+  double survival_ratio_ {3.0};    //!< Survival weight ratio
+  double weight_cutoff_ {DEFAULT_WEIGHT_CUTOFF}; //!< Weight cutoff
+  int max_split_ {10};             //!< Maximum value for particle splitting
+  int32_t mesh_idx_;               //!< index in meshes vector
+};
+
+} // namespace openmc
+#endif // OPENMC_WEIGHT_WINDOWS_H
diff --git a/openmc/__init__.py b/openmc/__init__.py
index d118e6a05..06a06aacc 100644
--- a/openmc/__init__.py
+++ b/openmc/__init__.py
@@ -11,6 +11,7 @@ from openmc.plots import *
 from openmc.region import *
 from openmc.volume import *
 from openmc.source import *
+from openmc.weight_windows import *
 from openmc.settings import *
 from openmc.surface import *
 from openmc.universe import *
diff --git a/openmc/mesh.py b/openmc/mesh.py
index 3cd208e89..2a16be1ad 100644
--- a/openmc/mesh.py
+++ b/openmc/mesh.py
@@ -88,6 +88,32 @@ class MeshBase(IDManagerMixin, ABC):
         else:
             raise ValueError('Unrecognized mesh type: "' + mesh_type + '"')
 
+    @classmethod
+    def from_xml(cls, elem):
+        """Generates a mesh from an XML element
+
+        Parameters
+        ----------
+        elem : xml.etree.ElementTree.Element
+            XML element
+
+        Returns
+        -------
+        openmc.MeshBase
+            an openmc mesh object
+
+        """
+        mesh_type = get_text(elem, 'type')
+
+        if mesh_type == 'regular' or mesh_type is None:
+            return RegularMesh.from_xml_element(elem)
+        elif mesh_type == 'rectilinear':
+            return RectilinearMesh.from_xml_element(elem)
+        elif mesh_type == 'unstructured':
+            return UnstructuredMesh.from_xml_element(elem)
+        else:
+            raise ValueError(f'Unrecognized mesh type "{mesh_type}" found.')
+
 
 class RegularMesh(MeshBase):
     """A regular Cartesian mesh in one, two, or three dimensions
@@ -305,7 +331,6 @@ class RegularMesh(MeshBase):
         if self._upper_right is not None:
             subelement = ET.SubElement(element, "upper_right")
             subelement.text = ' '.join(map(str, self._upper_right))
-
         if self._width is not None:
             subelement = ET.SubElement(element, "width")
             subelement.text = ' '.join(map(str, self._width))
@@ -574,19 +599,19 @@ class RectilinearMesh(MeshBase):
         fmt = '{0: <16}{1}{2}\n'
         string = super().__repr__()
         string += fmt.format('\tDimensions', '=\t', self.n_dimension)
-        x_grid_str = str(self._x_grid) if not self._x_grid else len(self._x_grid)
+        x_grid_str = str(self._x_grid) if self._x_grid is None else len(self._x_grid)
         string += fmt.format('\tN X pnts:', '=\t', x_grid_str)
-        if self._x_grid:
+        if self._x_grid is not None:
             string += fmt.format('\tX Min:', '=\t', self._x_grid[0])
             string += fmt.format('\tX Max:', '=\t', self._x_grid[-1])
-        y_grid_str = str(self._y_grid) if not self._y_grid else len(self._y_grid)
+        y_grid_str = str(self._y_grid) if self._y_grid is None else len(self._y_grid)
         string += fmt.format('\tN Y pnts:', '=\t', y_grid_str)
-        if self._y_grid:
+        if self._y_grid is not None:
             string += fmt.format('\tY Min:', '=\t', self._y_grid[0])
             string += fmt.format('\tY Max:', '=\t', self._y_grid[-1])
-        z_grid_str = str(self._z_grid) if not self._z_grid else len(self._z_grid)
+        z_grid_str = str(self._z_grid) if self._z_grid is None else len(self._z_grid)
         string += fmt.format('\tN Z pnts:', '=\t', z_grid_str)
-        if self._z_grid:
+        if self._z_grid is not None:
             string += fmt.format('\tZ Min:', '=\t', self._z_grid[0])
             string += fmt.format('\tZ Max:', '=\t', self._z_grid[-1])
         return string
@@ -603,6 +628,29 @@ class RectilinearMesh(MeshBase):
 
         return mesh
 
+    @classmethod
+    def from_xml_element(cls, elem):
+        """Generate a rectilinear mesh from an XML element
+
+        Parameters
+        ----------
+        elem : xml.etree.ElementTree.Element
+            XML element
+
+        Returns
+        -------
+        openmc.RectilinearMesh
+            Rectilinear mesh object
+
+        """
+        id = int(get_text(elem, 'id'))
+        mesh = cls(id)
+        mesh.x_grid = [float(x) for x in get_text(elem, 'x_grid').split()]
+        mesh.y_grid = [float(y) for y in get_text(elem, 'y_grid').split()]
+        mesh.z_grid = [float(z) for z in get_text(elem, 'z_grid').split()]
+
+        return mesh
+
     def to_xml_element(self):
         """Return XML representation of the mesh
 
@@ -1243,4 +1291,4 @@ class UnstructuredMesh(MeshBase):
         library = get_text(elem, 'library')
         length_multiplier = float(get_text(elem, 'length_multiplier', 1.0))
 
-        return cls(filename, library, mesh_id, '', length_multiplier)
+        return cls(filename, library, mesh_id, '', length_multiplier)
\ No newline at end of file
diff --git a/openmc/settings.py b/openmc/settings.py
index 6495acdd6..1ae159702 100644
--- a/openmc/settings.py
+++ b/openmc/settings.py
@@ -6,7 +6,7 @@ from xml.etree import ElementTree as ET
 from math import ceil
 
 import openmc.checkvalue as cv
-from . import VolumeCalculation, Source, RegularMesh
+from . import VolumeCalculation, Source, RegularMesh, WeightWindows
 from ._xml import clean_indentation, get_text, reorder_attributes
 
 
@@ -96,6 +96,10 @@ class Settings:
         .. versionadded:: 0.12
     max_order : None or int
         Maximum scattering order to apply globally when in multi-group mode.
+    max_splits : int
+        Maximum number of times a particle can split during a history
+
+        .. versionadded:: 0.13
     no_reduce : bool
         Indicate that all user-defined and global tallies should not be reduced
         across processes in a parallel calculation.
@@ -197,9 +201,16 @@ class Settings:
         described in :ref:`verbosity`.
     volume_calculations : VolumeCalculation or iterable of VolumeCalculation
         Stochastic volume calculation specifications
+    weight_windows : WeightWindows iterable of WeightWindows
+        Weight windows to use for variance reduction
+
+        .. versionadded:: 0.13
+    weight_windows_on : bool
+        Whether weight windows are enabled
+
+        .. versionadded:: 0.13
     write_initial_source : bool
         Indicate whether to write the initial source distribution to file
-
     """
 
     def __init__(self):
@@ -272,6 +283,9 @@ class Settings:
         self._event_based = None
         self._max_particles_in_flight = None
         self._write_initial_source = None
+        self._weight_windows = cv.CheckedList(WeightWindows, 'weight windows')
+        self._weight_windows_on = None
+        self._max_splits = None
 
     @property
     def run_mode(self):
@@ -445,6 +459,18 @@ class Settings:
     def write_initial_source(self):
         return self._write_initial_source
 
+    @property
+    def weight_windows(self):
+        return self._weight_windows
+
+    @property
+    def weight_windows_on(self):
+        return self._weight_windows_on
+
+    @property
+    def max_splits(self):
+        return self._max_splits
+
     @run_mode.setter
     def run_mode(self, run_mode):
         cv.check_value('run mode', run_mode, {x.value for x in RunMode})
@@ -835,6 +861,23 @@ class Settings:
         cv.check_type('write initial source', value, bool)
         self._write_initial_source = value
 
+    @weight_windows.setter
+    def weight_windows(self, value):
+        if not isinstance(value, MutableSequence):
+            value = [value]
+        self._weight_windows = cv.CheckedList(WeightWindows, 'weight windows', value)
+
+    @weight_windows_on.setter
+    def weight_windows_on(self, value):
+        cv.check_type('weight windows on', value, bool)
+        self._weight_windows_on = value
+
+    @max_splits.setter
+    def max_splits(self, value):
+        cv.check_type('maximum particle splits', value, Integral)
+        cv.check_greater_than('max particles in flight', value, 0)
+        self._max_splits = value
+
     def _create_run_mode_subelement(self, root):
         elem = ET.SubElement(root, "run_mode")
         elem.text = self._run_mode.value
@@ -1128,6 +1171,25 @@ class Settings:
             elem = ET.SubElement(root, "write_initial_source")
             elem.text = str(self._write_initial_source).lower()
 
+    def _create_weight_windows_subelement(self, root):
+        for ww in self._weight_windows:
+            # Add weight window information
+            root.append(ww.to_xml_element())
+
+            # See if a <mesh> element already exists -- if not, add it
+            path = f"./mesh[@id='{ww.mesh.id}']"
+            if root.find(path) is None:
+                root.append(ww.mesh.to_xml_element())
+
+        if self._weight_windows_on is not None:
+            elem = ET.SubElement(root, "weight_windows_on")
+            elem.text = str(self._weight_windows_on).lower()
+
+    def _create_max_splits_subelement(self, root):
+        if self._max_splits is not None:
+            elem = ET.SubElement(root, "max_splits")
+            elem.text = str(self._max_splits)
+
     def _eigenvalue_from_xml_element(self, root):
         elem = root.find('eigenvalue')
         if elem is not None:
@@ -1410,6 +1472,20 @@ class Settings:
         if text is not None:
             self.write_initial_source = text in ('true', '1')
 
+    def _weight_windows_from_xml_element(self, root):
+        for elem in root.findall('weight_windows'):
+            ww = WeightWindows.from_xml_element(elem, root)
+            self.weight_windows.append(ww)
+
+        text = get_text(root, 'weight_windows_on')
+        if text is not None:
+            self.weight_windows_on = text in ('true', '1')
+
+    def _max_splits_from_xml_element(self, root):
+        text = get_text(root, 'max_splits')
+        if text is not None:
+            self.max_splits = int(text)
+
     def export_to_xml(self, path='settings.xml'):
         """Export simulation settings to an XML file.
 
@@ -1464,6 +1540,8 @@ class Settings:
         self._create_material_cell_offsets_subelement(root_element)
         self._create_log_grid_bins_subelement(root_element)
         self._create_write_initial_source_subelement(root_element)
+        self._create_weight_windows_subelement(root_element)
+        self._create_max_splits_subelement(root_element)
 
         # Clean the indentation in the file to be user-readable
         clean_indentation(root_element)
@@ -1538,6 +1616,8 @@ class Settings:
         settings._material_cell_offsets_from_xml_element(root)
         settings._log_grid_bins_from_xml_element(root)
         settings._write_initial_source_from_xml_element(root)
+        settings._weight_windows_from_xml_element(root)
+        settings._max_splits_from_xml_element(root)
 
         # TODO: Get volume calculations
 
diff --git a/openmc/summary.py b/openmc/summary.py
index 6e6412061..41bae7f26 100644
--- a/openmc/summary.py
+++ b/openmc/summary.py
@@ -12,7 +12,7 @@ _VERSION_SUMMARY = 6
 
 
 class Summary:
-    """Summary of geometry, materials, and tallies used in a simulation.
+    """Summary of model used in a simulation.
 
     Attributes
     ----------
diff --git a/openmc/weight_windows.py b/openmc/weight_windows.py
new file mode 100644
index 000000000..ee8a4a2bf
--- /dev/null
+++ b/openmc/weight_windows.py
@@ -0,0 +1,334 @@
+from collections.abc import Iterable
+from numbers import Real, Integral
+
+from xml.etree import ElementTree as ET
+import numpy as np
+
+from openmc.filter import _PARTICLES
+from openmc.mesh import MeshBase
+import openmc.checkvalue as cv
+
+from ._xml import get_text
+from .mixin import IDManagerMixin
+
+
+class WeightWindows(IDManagerMixin):
+    """Mesh-based weight windows
+
+    This class enables you to specify weight window parameters that are used in
+    a simulation. Multiple sets of weight windows can be defined for different
+    meshes and different particles. An iterable of :class:`WeightWindows`
+    instances can be assigned to the :attr:`~openmc.Settings.weight_windows`
+    attribute, which is then exported to XML.
+
+    Weight window lower/upper bounds are to be specified for each combination of
+    a mesh element and an energy bin. Thus the total number of bounds should be
+    equal to the product of the number of mesh bins and the number of energy
+    bins.
+
+    .. versionadded:: 0.13
+
+    Parameters
+    ----------
+    mesh : openmc.MeshBase
+        Mesh for the weight windows
+    lower_ww_bounds : Iterable of Real
+        A list of values for which each value is the lower bound of a weight
+        window
+    upper_ww_bounds : Iterable of Real
+        A list of values for which each value is the upper bound of a weight
+        window
+    upper_bound_ratio : float
+        Ratio of the lower to upper weight window bounds
+    energy_bins : Iterable of Real
+        A list of values for which each successive pair constitutes a range of
+        energies in [eV] for a single bin
+    particle_type : {'neutron', 'photon'}
+        Particle type the weight windows apply to
+    survival_ratio : float
+        Ratio of the survival weight to the lower weight window bound for
+        rouletting
+    max_split : int
+        Maximum allowable number of particles when splitting
+    weight_cutoff : float
+        Threshold below which particles will be terminated
+    id : int
+       Unique identifier for the weight window settings. If not
+       specified an identifier will automatically be assigned.
+
+    Attributes
+    ----------
+    id : int
+       Unique identifier for the weight window settings.
+    mesh : openmc.MeshBase
+        Mesh for the weight windows
+    particle_type : str
+        Particle type the weight windows apply to
+    energy_bins : Iterable of Real
+        A list of values for which each successive pair constitutes a range of
+        energies in [eV] for a single bin
+    lower_ww_bounds : Iterable of Real
+        A list of values for which each value is the lower bound of a weight
+        window
+    upper_ww_bounds : Iterable of Real
+        A list of values for which each value is the upper bound of a weight
+        window
+    survival_ratio : float
+        Ratio of the survival weight to the lower weight window bound for
+        rouletting
+    max_split : int
+        Maximum allowable number of particles when splitting
+    weight_cutoff : float
+        Threshold below which particles will be terminated
+
+    See Also
+    --------
+    openmc.Settings
+
+    """
+    next_id = 1
+    used_ids = set()
+
+    def __init__(self, mesh, lower_ww_bounds, upper_ww_bounds=None,
+                 upper_bound_ratio=None, energy_bins=None, particle_type='neutron',
+                 survival_ratio=3, max_split=10, weight_cutoff=1.e-38, id=None):
+        self.mesh = mesh
+        self.id = id
+        self.particle_type = particle_type
+        self.energy_bins = energy_bins
+        self.lower_ww_bounds = lower_ww_bounds
+
+        cv.check_length('Lower window bounds', self.lower_ww_bounds, len(self.energy_bins))
+
+        if upper_ww_bounds is not None and upper_bound_ratio:
+            raise ValueError("Exactly one of uppwer_ww_bounds and "
+                             "upper_bound_ratio must be present.")
+
+        if upper_ww_bounds is None and upper_bound_ratio is None:
+            raise ValueError("Exactly one of uppwer_ww_bounds and "
+                             "upper_bound_ratio must be present.")
+
+        if upper_bound_ratio:
+            self.upper_ww_bounds = [
+                lb * upper_bound_ratio for lb in self.lower_ww_bounds
+            ]
+
+        if upper_ww_bounds is not None:
+            self.upper_ww_bounds = upper_ww_bounds
+
+        if len(self.lower_ww_bounds) != len(self.upper_ww_bounds):
+            raise ValueError('Size of the lower and upper weight window bounds'
+                             'do not match')
+
+        self.survival_ratio = survival_ratio
+        self.max_split = max_split
+        self.weight_cutoff = weight_cutoff
+
+    def __repr__(self):
+        string = type(self).__name__ + '\n'
+        string += '{: <16}=\t{}\n'.format('\tID', self._id)
+        string += '{: <16}=\t{}\n'.format('\tMesh:', self.mesh)
+        string += '{: <16}=\t{}\n'.format('\tParticle Type', self._particle_type)
+        string += '{: <16}=\t{}\n'.format('\tEnergy Bins', self._energy_bins)
+        string += '{: <16}=\t{}\n'.format('\tLower WW Bounds', self._lower_ww_bounds)
+        string += '{: <16}=\t{}\n'.format('\tUpper WW Bounds', self._upper_ww_bounds)
+        string += '{: <16}=\t{}\n'.format('\tSurvival Ratio', self._survival_ratio)
+        string += '{: <16}=\t{}\n'.format('\tMax Split', self._max_split)
+        string += '{: <16}=\t{}\n'.format('\tWeight Cutoff', self._weight_cutoff)
+        return string
+
+    @property
+    def mesh(self):
+        return self._mesh
+
+    @mesh.setter
+    def mesh(self, mesh):
+        cv.check_type('Weight window mesh', mesh, MeshBase)
+        self._mesh = mesh
+
+    @property
+    def particle_type(self):
+        return self._particle_type
+
+    @particle_type.setter
+    def particle_type(self, pt):
+        cv.check_value('Particle type', pt, _PARTICLES)
+        self._particle_type = pt
+
+    @property
+    def energy_bins(self):
+        return self._energy_bins
+
+    @energy_bins.setter
+    def energy_bins(self, bins):
+        cv.check_type('Energy bins', bins, Iterable, Real)
+        self._energy_bins = np.array(bins)
+
+    @property
+    def lower_ww_bounds(self):
+        return self._lower_ww_bounds
+
+    @lower_ww_bounds.setter
+    def lower_ww_bounds(self, bounds):
+        cv.check_type('Lower WW bounds', bounds, Iterable, Real)
+        self._lower_ww_bounds = np.array(bounds)
+
+    @property
+    def upper_ww_bounds(self):
+        return self._upper_ww_bounds
+
+    @upper_ww_bounds.setter
+    def upper_ww_bounds(self, bounds):
+        cv.check_type('Upper WW bounds', bounds, Iterable, Real)
+        self._upper_ww_bounds = np.array(bounds)
+
+    @property
+    def survival_ratio(self):
+        return self._survival_ratio
+
+    @survival_ratio.setter
+    def survival_ratio(self, val):
+        cv.check_type('Survival ratio', val, Real)
+        cv.check_greater_than('Survival ratio', val, 1.0, True)
+        self._survival_ratio = val
+
+    @property
+    def max_split(self):
+        return self._max_split
+
+    @max_split.setter
+    def max_split(self, val):
+        cv.check_type('Max split', val, Integral)
+        self._max_split = val
+
+    @property
+    def weight_cutoff(self):
+        return self._weight_cutoff
+
+    @weight_cutoff.setter
+    def weight_cutoff(self, cutoff):
+        cv.check_type('Weight cutoff', cutoff, Real)
+        cv.check_greater_than('Weight cutoff', cutoff, 0.0, True)
+        self._weight_cutoff = cutoff
+
+    def to_xml_element(self):
+        """Return an XML representation of the weight window settings
+
+        Returns
+        -------
+        element : xml.etree.ElementTree.Element
+            XML element containing the weight window information
+        """
+        element = ET.Element('weight_windows')
+
+        element.set('id', str(self._id))
+
+        subelement = ET.SubElement(element, 'mesh')
+        subelement.text = str(self.mesh.id)
+
+        subelement = ET.SubElement(element, 'particle_type')
+        subelement.text = self.particle_type
+
+        subelement = ET.SubElement(element, 'energy_bins')
+        subelement.text = ' '.join(str(e) for e in self.energy_bins)
+
+        subelement = ET.SubElement(element, 'lower_ww_bounds')
+        subelement.text = ' '.join(str(b) for b in self.lower_ww_bounds)
+
+        subelement = ET.SubElement(element, 'upper_ww_bounds')
+        subelement.text = ' '.join(str(b) for b in self.upper_ww_bounds)
+
+        subelement = ET.SubElement(element, 'survival_ratio')
+        subelement.text = str(self.survival_ratio)
+
+        subelement = ET.SubElement(element, 'max_split')
+        subelement.text = str(self.max_split)
+
+        subelement = ET.SubElement(element, 'weight_cutoff')
+        subelement.text = str(self.weight_cutoff)
+
+        return element
+
+    @classmethod
+    def from_xml_element(cls, elem, root):
+        """Generate weight window settings from an XML element
+
+        Parameters
+        ----------
+        elem : xml.etree.ElementTree.Element
+            XML element
+        root : xml.etree.ElementTree.Element
+            Root element for the file where meshes can be found
+
+        Returns
+        -------
+        openmc.WeightWindows
+            Weight windows object
+        """
+        # Get mesh for weight windows
+        mesh_id = int(get_text(elem, 'mesh'))
+        path = f"./mesh[@id='{mesh_id}']"
+        mesh_elem = root.find(path)
+        if mesh_elem is not None:
+            mesh = MeshBase.from_xml(mesh_elem)
+
+        # Read all other parameters
+        lower_ww_bounds = [float(l) for l in get_text(elem, 'lower_ww_bounds').split()]
+        upper_ww_bounds = [float(u) for u in get_text(elem, 'upper_ww_bounds').split()]
+        ebins = [float(b) for b in get_text(elem, 'energy_bins').split()]
+        particle_type = get_text(elem, 'particle_type')
+        survival_ratio = float(get_text(elem, 'survival_ratio'))
+        max_split = int(get_text(elem, 'max_split'))
+        weight_cutoff = float(get_text(elem, 'weight_cutoff'))
+        id = int(get_text(elem, 'id'))
+
+        return cls(
+            mesh=mesh,
+            lower_ww_bounds=lower_ww_bounds,
+            upper_ww_bounds=upper_ww_bounds,
+            energy_bins=ebins,
+            particle_type=particle_type,
+            survival_ratio=survival_ratio,
+            max_split=max_split,
+            weight_cutoff=weight_cutoff,
+            id=id
+        )
+
+    @classmethod
+    def from_hdf5(cls, group, meshes):
+        """Create weight windows from HDF5 group
+
+        Parameters
+        ----------
+        group : h5py.Group
+            Group in HDF5 file
+        meshes : dict
+            Dictionary mapping IDs to mesh objects
+
+        Returns
+        -------
+        openmc.WeightWindows
+            A weight window object
+        """
+
+        id = int(group.name.split('/')[-1].lstrip('weight_windows'))
+        mesh_id = group['mesh'][()]
+        ptype = group['particle_type'][()].decode()
+        ebins = group['energy_bins'][()]
+        lower_ww_bounds = group['lower_ww_bounds'][()]
+        upper_ww_bounds = group['upper_ww_bounds'][()]
+        survival_ratio = group['survival_ratio'][()]
+        max_split = group['max_split'][()]
+        weight_cutoff = group['weight_cutoff'][()]
+
+        return cls(
+            mesh=meshes[mesh_id],
+            lower_ww_bounds=lower_ww_bounds,
+            upper_ww_bounds=upper_ww_bounds,
+            energy_bins=ebins,
+            particle_type=ptype,
+            survival_ratio=survival_ratio,
+            max_split=max_split,
+            weight_cutoff=weight_cutoff,
+            id=id
+        )
diff --git a/src/finalize.cpp b/src/finalize.cpp
index 2f6c65aaf..65880c0b8 100644
--- a/src/finalize.cpp
+++ b/src/finalize.cpp
@@ -25,6 +25,7 @@
 #include "openmc/thermal.h"
 #include "openmc/timer.h"
 #include "openmc/volume_calc.h"
+#include "openmc/weight_windows.h"
 
 #include "xtensor/xview.hpp"
 
@@ -47,6 +48,7 @@ void free_memory()
   free_memory_tally();
   free_memory_bank();
   free_memory_plot();
+  free_memory_weight_windows();
   if (mpi::master) {
     free_memory_cmfd();
   }
@@ -82,6 +84,7 @@ int openmc_finalize()
   settings::legendre_to_tabular_points = -1;
   settings::material_cell_offsets = true;
   settings::max_particles_in_flight = 100000;
+  settings::max_splits = 1000;
   settings::n_inactive = 0;
   settings::n_particles = -1;
   settings::output_summary = true;
@@ -113,6 +116,7 @@ int openmc_finalize()
   settings::verbosity = 7;
   settings::weight_cutoff = 0.25;
   settings::weight_survive = 1.0;
+  settings::weight_windows_on = false;
   settings::write_all_tracks = false;
   settings::write_initial_source = false;
 
diff --git a/src/physics.cpp b/src/physics.cpp
index bb9873bd1..196c7dc3b 100644
--- a/src/physics.cpp
+++ b/src/physics.cpp
@@ -23,6 +23,7 @@
 #include "openmc/string_utils.h"
 #include "openmc/tallies/tally.h"
 #include "openmc/thermal.h"
+#include "openmc/weight_windows.h"
 
 #include <fmt/core.h>
 
@@ -44,9 +45,15 @@ void collision(Particle& p)
   switch (p.type()) {
   case ParticleType::neutron:
     sample_neutron_reaction(p);
+    if (settings::weight_windows_on) {
+      apply_weight_windows(p);
+    }
     break;
   case ParticleType::photon:
     sample_photon_reaction(p);
+    if (settings::weight_windows_on) {
+      apply_weight_windows(p);
+    }
     break;
   case ParticleType::electron:
     sample_electron_reaction(p);
diff --git a/src/settings.cpp b/src/settings.cpp
index 37762517b..795b164ec 100644
--- a/src/settings.cpp
+++ b/src/settings.cpp
@@ -27,6 +27,7 @@
 #include "openmc/string_utils.h"
 #include "openmc/tallies/trigger.h"
 #include "openmc/volume_calc.h"
+#include "openmc/weight_windows.h"
 #include "openmc/xml_interface.h"
 
 namespace openmc {
@@ -67,6 +68,7 @@ bool trigger_on {false};
 bool trigger_predict {false};
 bool ufs_on {false};
 bool urr_ptables_on {true};
+bool weight_windows_on {false};
 bool write_all_tracks {false};
 bool write_initial_source {false};
 
@@ -92,6 +94,7 @@ int max_order {0};
 int n_log_bins {8000};
 int n_batches;
 int n_max_batches;
+int max_splits {1000};
 ResScatMethod res_scat_method {ResScatMethod::rvs};
 double res_scat_energy_min {0.01};
 double res_scat_energy_max {1000.0};
@@ -851,6 +854,23 @@ void read_settings_xml()
   if (check_for_node(root, "material_cell_offsets")) {
     material_cell_offsets = get_node_value_bool(root, "material_cell_offsets");
   }
+
+  // Weight window information
+  for (pugi::xml_node node_ww : root.children("weight_windows")) {
+    variance_reduction::weight_windows.emplace_back(
+      std::make_unique<WeightWindows>(node_ww));
+
+    // Enable weight windows by default if one or more are present
+    settings::weight_windows_on = true;
+  }
+
+  if (check_for_node(root, "weight_windows_on")) {
+    weight_windows_on = get_node_value_bool(root, "weight_windows_on");
+  }
+
+  if (check_for_node(root, "max_splits")) {
+    settings::max_splits = std::stoi(get_node_value(root, "max_splits"));
+  }
 }
 
 void free_memory_settings()
diff --git a/src/simulation.cpp b/src/simulation.cpp
index 35d76ed87..772c90950 100644
--- a/src/simulation.cpp
+++ b/src/simulation.cpp
@@ -487,6 +487,9 @@ void initialize_history(Particle& p, int64_t index_source)
   // Reset particle event counter
   p.n_event() = 0;
 
+  // Reset split counter
+  p.n_split() = 0;
+
   // set random number seed
   int64_t particle_seed =
     (simulation::total_gen + overall_generation() - 1) * settings::n_particles +
diff --git a/src/weight_windows.cpp b/src/weight_windows.cpp
new file mode 100644
index 000000000..c3859d7d1
--- /dev/null
+++ b/src/weight_windows.cpp
@@ -0,0 +1,264 @@
+#include "openmc/weight_windows.h"
+
+#include "openmc/error.h"
+#include "openmc/file_utils.h"
+#include "openmc/hdf5_interface.h"
+#include "openmc/particle.h"
+#include "openmc/particle_data.h"
+#include "openmc/search.h"
+#include "openmc/xml_interface.h"
+
+#include <fmt/core.h>
+#include <gsl/gsl-lite.hpp>
+
+namespace openmc {
+
+//==============================================================================
+// Global variables
+//==============================================================================
+
+namespace variance_reduction {
+
+std::unordered_map<int32_t, int32_t> ww_map;
+openmc::vector<unique_ptr<WeightWindows>> weight_windows;
+
+} // namespace variance_reduction
+
+//==============================================================================
+// Non-member functions
+//==============================================================================
+
+void apply_weight_windows(Particle& p)
+{
+  // skip dead or no energy
+  if (p.E() <= 0 || !p.alive())
+    return;
+
+  bool in_domain = false;
+  // TODO: this is a linear search - should do something more clever
+  WeightWindow weight_window;
+  for (const auto& ww : variance_reduction::weight_windows) {
+    weight_window = ww->get_weight_window(p);
+    if (weight_window.is_valid())
+      break;
+  }
+  // particle is not in any of the ww domains, do nothing
+  if (!weight_window.is_valid())
+    return;
+
+  // get the paramters
+  double weight = p.wgt();
+
+  // first check to see if particle should be killed for weight cutoff
+  if (p.wgt() < weight_window.weight_cutoff) {
+    p.alive() = false;
+    p.wgt() = 0.0;
+    return;
+  }
+
+  // if particle's weight is above the weight window split until they are within
+  // the window
+  if (weight > weight_window.upper_weight) {
+    // do not further split the particle if above the limit
+    if (p.n_split() >= settings::max_splits)
+      return;
+
+    double n_split = std::ceil(weight / weight_window.upper_weight);
+    double max_split = weight_window.max_split;
+    n_split = std::min(n_split, max_split);
+
+    p.n_split() += n_split;
+
+    // Create secondaries and divide weight among all particles
+    int i_split = std::round(n_split);
+    for (int l = 0; l < i_split - 1; l++) {
+      p.create_secondary(weight / n_split, p.u(), p.E(), p.type());
+    }
+    // remaining weight is applied to current particle
+    p.wgt() = weight / n_split;
+
+  } else if (weight <= weight_window.lower_weight) {
+    // if the particle weight is below the window, play Russian roulette
+    double weight_survive =
+      std::min(weight * weight_window.max_split, weight_window.survival_weight);
+    if (weight_survive * prn(p.current_seed()) <= weight) {
+      p.wgt() = weight_survive;
+    } else {
+      p.alive() = false;
+      p.wgt() = 0.0;
+    }
+    // else particle is in the window, continue as normal
+  }
+}
+
+void free_memory_weight_windows()
+{
+  variance_reduction::ww_map.clear();
+  variance_reduction::weight_windows.clear();
+}
+
+//==============================================================================
+// WeightWindowSettings implementation
+//==============================================================================
+
+WeightWindows::WeightWindows(pugi::xml_node node)
+{
+  // Make sure required elements are present
+  const vector<std::string> required_elems {
+    "id", "particle_type", "energy_bins", "lower_ww_bounds", "upper_ww_bounds"};
+  for (const auto& elem : required_elems) {
+    if (!check_for_node(node, elem.c_str())) {
+      fatal_error(fmt::format("Must specify <{}> for weight windows.", elem));
+    }
+  }
+
+  // Get weight windows ID
+  int32_t id = std::stoi(get_node_value(node, "id"));
+  this->set_id(id);
+
+  // get the particle type
+  auto particle_type_str = std::string(get_node_value(node, "particle_type"));
+  particle_type_ = openmc::str_to_particle_type(particle_type_str);
+
+  // Determine associated mesh
+  int32_t mesh_id = std::stoi(get_node_value(node, "mesh"));
+  mesh_idx_ = model::mesh_map.at(mesh_id);
+
+  // energy bounds
+  energy_bins_ = get_node_array<double>(node, "energy_bins");
+
+  // read the lower/upper weight bounds
+  lower_ww_ = get_node_array<double>(node, "lower_ww_bounds");
+  upper_ww_ = get_node_array<double>(node, "upper_ww_bounds");
+
+  // get the survival value - optional
+  if (check_for_node(node, "survival_ratio")) {
+    survival_ratio_ = std::stod(get_node_value(node, "survival_ratio"));
+    if (survival_ratio_ <= 1)
+      fatal_error("Survival to lower weight window ratio must bigger than 1 "
+                  "and less than the upper to lower weight window ratio.");
+  }
+
+  // get the max split - optional
+  if (check_for_node(node, "max_split")) {
+    max_split_ = std::stod(get_node_value(node, "max_split"));
+    if (max_split_ <= 1)
+      fatal_error("max split must be larger than 1");
+  }
+
+  // weight cutoff - optional
+  if (check_for_node(node, "weight_cutoff")) {
+    weight_cutoff_ = std::stod(get_node_value(node, "weight_cutoff"));
+    if (weight_cutoff_ <= 0)
+      fatal_error("weight_cutoff must be larger than 0");
+    if (weight_cutoff_ > 1)
+      fatal_error("weight_cutoff must be less than 1");
+  }
+
+  // make sure that the upper and lower bounds have the same size
+  if (upper_ww_.size() != lower_ww_.size()) {
+    fatal_error("The upper and lower weight window lengths do not match.");
+  }
+
+  // num spatial*energy bins must match num weight bins
+  int num_spatial_bins = this->mesh().n_bins();
+  int num_energy_bins = energy_bins_.size() - 1;
+  int num_weight_bins = lower_ww_.size();
+  if (num_weight_bins != num_spatial_bins * num_energy_bins) {
+    auto err_msg =
+      fmt::format("In weight window domain {} the number of spatial "
+                  "energy/spatial bins ({}) does not match the number "
+                  "of weight bins ({})",
+        id_, num_energy_bins, num_weight_bins);
+    fatal_error(err_msg);
+  }
+}
+
+void WeightWindows::set_id(int32_t id)
+{
+  Expects(id >= 0 || id == C_NONE);
+
+  // Clear entry in mesh map in case one was already assigned
+  if (id_ != C_NONE) {
+    variance_reduction::ww_map.erase(id_);
+    id_ = C_NONE;
+  }
+
+  // Ensure no other mesh has the same ID
+  if (variance_reduction::ww_map.find(id) != variance_reduction::ww_map.end()) {
+    throw std::runtime_error {
+      fmt::format("Two weight windows have the same ID: {}", id)};
+  }
+
+  // If no ID is specified, auto-assign the next ID in the sequence
+  if (id == C_NONE) {
+    id = 0;
+    for (const auto& m : variance_reduction::weight_windows) {
+      id = std::max(id, m->id_);
+    }
+    ++id;
+  }
+
+  // Update ID and entry in the mesh map
+  id_ = id;
+  variance_reduction::ww_map[id] =
+    variance_reduction::weight_windows.size() - 1;
+}
+
+WeightWindow WeightWindows::get_weight_window(const Particle& p) const
+{
+  // check for particle type
+  if (particle_type_ != p.type()) {
+    return {};
+  }
+
+  // Get mesh index for particle's position
+  const auto& mesh = this->mesh();
+  int ww_index = mesh.get_bin(p.r());
+
+  // particle is outside the weight window mesh
+  if (ww_index < 0)
+    return {};
+
+  // particle energy
+  double E = p.E();
+
+  // check to make sure energy is in range, expects sorted energy values
+  if (E < energy_bins_.front() || E > energy_bins_.back())
+    return {};
+
+  // get the mesh bin in energy group
+  int energy_bin =
+    lower_bound_index(energy_bins_.begin(), energy_bins_.end(), E);
+
+  // indices now points to the correct weight for the given energy
+  ww_index += energy_bin * mesh.n_bins();
+
+  // Create individual weight window
+  WeightWindow ww;
+  ww.lower_weight = lower_ww_[ww_index];
+  ww.upper_weight = upper_ww_[ww_index];
+  ww.survival_weight = ww.lower_weight * survival_ratio_;
+  ww.max_split = max_split_;
+  ww.weight_cutoff = weight_cutoff_;
+  return ww;
+}
+
+void WeightWindows::to_hdf5(hid_t group) const
+{
+  hid_t ww_group = create_group(group, fmt::format("weight_windows {}", id_));
+
+  write_dataset(
+    ww_group, "particle_type", openmc::particle_type_to_str(particle_type_));
+  write_dataset(ww_group, "energy_bins", energy_bins_);
+  write_dataset(ww_group, "lower_ww_bounds", lower_ww_);
+  write_dataset(ww_group, "upper_ww_bounds", upper_ww_);
+  write_dataset(ww_group, "survival_ratio", survival_ratio_);
+  write_dataset(ww_group, "max_split", max_split_);
+  write_dataset(ww_group, "weight_cutoff", weight_cutoff_);
+  write_dataset(ww_group, "mesh", this->mesh().id_);
+
+  close_group(ww_group);
+}
+
+} // namespace openmc
diff --git a/tests/__init__.py b/tests/__init__.py
index 5c21cd938..0228348a2 100644
--- a/tests/__init__.py
+++ b/tests/__init__.py
@@ -1,13 +1,23 @@
 from contextlib import contextmanager
 import os
+from shutil import copy
 import tempfile
 
 
 @contextmanager
-def cdtemp():
-    """Context manager to change to/return from a tmpdir."""
+def cdtemp(files=None):
+    """Context manager to change to/return from a tmpdir.
+
+    Parameters
+    ----------
+    files : Iterable of str or Path-like
+        Set of files to copy into the temporary directory
+    """
     with tempfile.TemporaryDirectory() as tmpdir:
         cwd = os.getcwd()
+        if files:
+            for file in files:
+                copy(file, tmpdir, follow_symlinks=True)
         try:
             os.chdir(tmpdir)
             yield
diff --git a/tests/regression_tests/weightwindows/__init__.py b/tests/regression_tests/weightwindows/__init__.py
new file mode 100644
index 000000000..8b1378917
--- /dev/null
+++ b/tests/regression_tests/weightwindows/__init__.py
@@ -0,0 +1 @@
+
diff --git a/tests/regression_tests/weightwindows/inputs_true.dat b/tests/regression_tests/weightwindows/inputs_true.dat
new file mode 100644
index 000000000..5459a09b9
--- /dev/null
+++ b/tests/regression_tests/weightwindows/inputs_true.dat
@@ -0,0 +1,94 @@
+<?xml version='1.0' encoding='utf-8'?>
+<geometry>
+  <cell id="1" material="1" region="-1" universe="1" />
+  <cell id="2" material="void" region="1 -2" universe="1" />
+  <surface coeffs="0.0 0.0 0.0 240" id="1" type="sphere" />
+  <surface boundary="vacuum" coeffs="0.0 0.0 0.0 250" id="2" type="sphere" />
+</geometry>
+<?xml version='1.0' encoding='utf-8'?>
+<materials>
+  <material id="1">
+    <density units="g/cc" value="2.3" />
+    <nuclide ao="0.168018676" name="H1" />
+    <nuclide ao="1.93244e-05" name="H2" />
+    <nuclide ao="0.561814465" name="O16" />
+    <nuclide ao="0.00021401" name="O17" />
+    <nuclide ao="0.021365" name="Na23" />
+    <nuclide ao="0.021343" name="Al27" />
+    <nuclide ao="0.187439342" name="Si28" />
+    <nuclide ao="0.009517714" name="Si29" />
+    <nuclide ao="0.006273944" name="Si30" />
+    <nuclide ao="0.018026179" name="Ca40" />
+    <nuclide ao="0.00012031" name="Ca42" />
+    <nuclide ao="2.51033e-05" name="Ca43" />
+    <nuclide ao="0.000387892" name="Ca44" />
+    <nuclide ao="7.438e-07" name="Ca46" />
+    <nuclide ao="3.47727e-05" name="Ca48" />
+    <nuclide ao="0.000248179" name="Fe54" />
+    <nuclide ao="0.003895875" name="Fe56" />
+    <nuclide ao="8.99727e-05" name="Fe57" />
+    <nuclide ao="1.19737e-05" name="Fe58" />
+  </material>
+</materials>
+<?xml version='1.0' encoding='utf-8'?>
+<settings>
+  <run_mode>fixed source</run_mode>
+  <particles>200</particles>
+  <batches>2</batches>
+  <source strength="1.0">
+    <space type="point">
+      <parameters>0.001 0.001 0.001</parameters>
+    </space>
+    <energy type="discrete">
+      <parameters>14000000.0 1.0</parameters>
+    </energy>
+  </source>
+  <photon_transport>true</photon_transport>
+  <weight_windows id="1">
+    <mesh>2</mesh>
+    <particle_type>neutron</particle_type>
+    <energy_bins>0.0 0.5 20000000.0</energy_bins>
+    <lower_ww_bounds>-1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 7.695031155172816e-14 -1.0 7.476545089278482e-06 -1.0 2.1612150425221703e-06 -1.0 -1.0 -1.0 2.1360401784344975e-18 -1.0 1.3455341306162382e-05 -1.0 3.353295403842037e-05 -1.0 3.0160758454323657e-07 -1.0 1.0262431550731535e-13 -1.0 1.6859219614058024e-19 -1.0 3.853097455417877e-06 -1.0 4.354946981329799e-05 -1.0 7.002861620919232e-08 -1.0 -1.0 -1.0 -1.0 -1.0 1.6205064883026195e-11 -1.0 1.7041669224797384e-09 -1.0 1.8747824667478637e-14 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 5.619835883512353e-14 -1.0 1.951356547084204e-15 -1.0 8.48176145671274e-10 -1.0 -1.0 -1.0 1.4508262073256659e-13 -1.0 4.109066205029878e-05 -1.0 0.00020573181450240786 -1.0 2.124999182004655e-05 -1.0 5.5379154134462336e-08 -1.0 1.5550455959178486e-06 -1.0 0.0007213722022489493 -1.0 0.007673145137236567 -1.0 0.0008175033526488423 -1.0 1.008394964750947e-06 -1.0 6.848642863465585e-07 -1.0 0.0010737081832502356 -1.0 0.007197662162700777 -1.0 0.0007151459162818186 -1.0 2.253382683081525e-06 -1.0 1.3181562352445092e-10 -1.0 1.93847081892941e-05 -1.0 0.00046029617115127556 -1.0 2.5629068330446215e-05 -1.0 5.504813693036933e-12 -1.0 -1.0 -1.0 3.3164825349503764e-16 -1.0 3.937160538176268e-07 -1.0 -1.0 -1.0 -1.0 -1.0 1.5172912057312398e-14 -1.0 6.315154082213375e-07 -1.0 3.855884234344845e-06 -1.0 1.7770136411255912e-05 -1.0 1.6421491993751715e-11 -1.0 5.97653814110781e-06 -1.0 0.001959411999677113 -1.0 0.013330619853379314 -1.0 0.0017950613169359904 -1.0 2.4306375693722205e-06 -1.0 6.922231352729155e-05 -1.0 0.08657573566388353 -1.0 -1.0 -1.0 0.0835648710074336 -1.0 0.00016419031150495913 -1.0 9.03108551826469e-05 -1.0 0.0848872967381878 -1.0 -1.0 -1.0 0.08288806458368345 -1.0 5.6949597066784976e-05 -1.0 8.037131813528501e-07 -1.0 0.0016637126543321873 -1.0 0.01664109232689093 -1.0 0.0018327593437608 -1.0 6.542700644645627e-07 -1.0 -1.0 -1.0 6.655926357459275e-08 -1.0 5.783974359937857e-06 -1.0 2.679340942769232e-06 -1.0 -1.0 -1.0 -1.0 -1.0 2.1040609012865134e-05 -1.0 2.9282097947816224e-05 -1.0 1.284759166582202e-05 -1.0 1.0092905083158804e-11 -1.0 1.3228013765525015e-05 -1.0 0.007136750029575816 -1.0 0.06539005235506369 -1.0 0.0064803314120165335 -1.0 2.6011787393916806e-06 -1.0 8.887234042637684e-05 -1.0 0.5 -1.0 -1.0 -1.0 0.4877856021170283 -1.0 0.00017699858357744463 -1.0 0.0001670700632870335 -1.0 0.4899999304038166 -1.0 -1.0 -1.0 0.48573842213878143 -1.0 0.00021534568699157805 -1.0 7.84289689494925e-06 -1.0 0.0063790654507599135 -1.0 0.06550426960512012 -1.0 0.006639379668946672 -1.0 7.754700849337902e-06 -1.0 2.7207433165796702e-11 -1.0 5.985108617124989e-06 -1.0 3.634644995026692e-05 -1.0 3.932389299316285e-06 -1.0 -1.0 -1.0 -1.0 -1.0 1.271128669411295e-07 -1.0 8.846493399850663e-06 -1.0 2.499931357628383e-07 -1.0 8.533030345699353e-17 -1.0 1.6699254277734109e-06 -1.0 0.0017541380096893788 -1.0 0.015148978428271613 -1.0 0.0016279794552427574 -1.0 2.380955631371226e-06 -1.0 2.1648918040467363e-05 -1.0 0.0833931797381589 -1.0 -1.0 -1.0 0.08285899874014539 -1.0 2.762955748645507e-05 -1.0 5.1161740516205715e-05 -1.0 0.08095280568305474 -1.0 -1.0 -1.0 0.08636569925236791 -1.0 3.452537179033895e-05 -1.0 3.5981031766416143e-06 -1.0 0.001570829313580841 -1.0 0.01621821782442112 -1.0 0.0015447823995470042 -1.0 7.989579285134734e-06 -1.0 -1.0 -1.0 2.594951085377588e-06 -1.0 4.323138781337963e-05 -1.0 6.948292975998466e-07 -1.0 -1.0 -1.0 -1.0 -1.0 2.4469456008493614e-09 -1.0 9.041149893307806e-07 -1.0 2.2060992580622125e-11 -1.0 -1.0 -1.0 1.681305446488884e-11 -1.0 7.091263906557291e-05 -1.0 0.00027464244062887683 -1.0 1.28584557092134e-05 -1.0 2.0295213621716706e-09 -1.0 1.2911827198500372e-08 -1.0 0.0009127050763987511 -1.0 0.007385384405891666 -1.0 0.0010019432401508087 -1.0 1.080711295768551e-05 -1.0 8.020767115181574e-07 -1.0 0.0009252532821720597 -1.0 0.007188410694198128 -1.0 0.0007245451610090619 -1.0 2.5157355790201773e-07 -1.0 1.4825219199133353e-12 -1.0 4.756872959630257e-05 -1.0 0.0002668579837809081 -1.0 2.8226276944532696e-05 -1.0 5.182269672663266e-10 -1.0 -1.0 -1.0 2.936268747135548e-14 -1.0 2.5447281241730366e-07 -1.0 3.89841667138598e-08 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 2.639268952045579e-10 -1.0 3.7051518220083886e-06 -1.0 1.1333627604823673e-09 -1.0 -1.0 -1.0 -1.0 -1.0 1.2296747260635405e-06 -1.0 7.962899789920809e-06 -1.0 8.211982683649991e-09 -1.0 -1.0 -1.0 -1.0 -1.0 2.5639656866250453e-06 -1.0 2.2487617828938326e-05 -1.0 3.2799045883372075e-06 -1.0 4.076762579823379e-17 -1.0 1.5476769237571295e-14 -1.0 1.0305672698745657e-11 -1.0 1.957092814065688e-05 -1.0 3.8523247550378815e-12 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 2.7697233171399416e-13 -1.0 -1.0 -1.0 -1.0 -1.0</lower_ww_bounds>
+    <upper_ww_bounds>-10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 7.695031155172816e-13 -10.0 7.476545089278483e-05 -10.0 2.1612150425221703e-05 -10.0 -10.0 -10.0 2.1360401784344976e-17 -10.0 0.00013455341306162382 -10.0 0.00033532954038420373 -10.0 3.0160758454323656e-06 -10.0 1.0262431550731535e-12 -10.0 1.6859219614058023e-18 -10.0 3.853097455417877e-05 -10.0 0.00043549469813297995 -10.0 7.002861620919232e-07 -10.0 -10.0 -10.0 -10.0 -10.0 1.6205064883026195e-10 -10.0 1.7041669224797384e-08 -10.0 1.8747824667478637e-13 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 5.619835883512353e-13 -10.0 1.951356547084204e-14 -10.0 8.48176145671274e-09 -10.0 -10.0 -10.0 1.4508262073256658e-12 -10.0 0.0004109066205029878 -10.0 0.0020573181450240785 -10.0 0.00021249991820046548 -10.0 5.537915413446234e-07 -10.0 1.5550455959178486e-05 -10.0 0.0072137220224894934 -10.0 0.07673145137236567 -10.0 0.008175033526488424 -10.0 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-10.0 0.0001080711295768551 -10.0 8.020767115181574e-06 -10.0 0.009252532821720597 -10.0 0.07188410694198127 -10.0 0.0072454516100906195 -10.0 2.515735579020177e-06 -10.0 1.4825219199133352e-11 -10.0 0.0004756872959630257 -10.0 0.0026685798378090807 -10.0 0.000282262769445327 -10.0 5.182269672663266e-09 -10.0 -10.0 -10.0 2.9362687471355483e-13 -10.0 2.5447281241730365e-06 -10.0 3.89841667138598e-07 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 2.639268952045579e-09 -10.0 3.7051518220083885e-05 -10.0 1.1333627604823672e-08 -10.0 -10.0 -10.0 -10.0 -10.0 1.2296747260635405e-05 -10.0 7.96289978992081e-05 -10.0 8.211982683649991e-08 -10.0 -10.0 -10.0 -10.0 -10.0 2.5639656866250452e-05 -10.0 0.00022487617828938325 -10.0 3.2799045883372076e-05 -10.0 4.076762579823379e-16 -10.0 1.5476769237571296e-13 -10.0 1.0305672698745658e-10 -10.0 0.0001957092814065688 -10.0 3.852324755037882e-11 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 2.7697233171399415e-12 -10.0 -10.0 -10.0 -10.0 -10.0</upper_ww_bounds>
+    <survival_ratio>3</survival_ratio>
+    <max_split>10</max_split>
+    <weight_cutoff>1e-38</weight_cutoff>
+  </weight_windows>
+  <mesh id="2">
+    <dimension>5 6 7</dimension>
+    <lower_left>-240 -240 -240</lower_left>
+    <upper_right>240 240 240</upper_right>
+  </mesh>
+  <weight_windows id="2">
+    <mesh>2</mesh>
+    <particle_type>neutron</particle_type>
+    <energy_bins>0.0 0.5 20000000.0</energy_bins>
+    <lower_ww_bounds>-1.0 -1.0 -1.0 5.447493368067179e-10 -1.0 4.0404626147344e-13 -1.0 9.382800796426366e-17 -1.0 -1.0 -1.0 5.752120832330886e-11 -1.0 3.520674728719278e-05 -1.0 8.76041935303527e-05 -1.0 3.136829920007009e-05 -1.0 -1.0 -1.0 4.233410073487772e-06 -1.0 0.00025519632243117644 -1.0 0.0004777967757842694 -1.0 0.00035983547999740665 -1.0 4.6258592060517866e-07 -1.0 4.587143797469485e-08 -1.0 0.00012918433677593975 -1.0 0.000469589568463265 -1.0 8.862760180332873e-05 -1.0 3.2790914744011534e-06 -1.0 1.1514579117272292e-10 -1.0 2.2950716170641463e-05 -1.0 6.92296299242814e-05 -1.0 2.1606550732170693e-05 -1.0 2.617878055106949e-16 -1.0 -1.0 -1.0 -1.0 -1.0 1.722191123879782e-08 -1.0 -1.0 -1.0 -1.0 -1.0 2.1234980618763222e-13 -1.0 1.4628847959717225e-05 -1.0 2.777791741571174e-05 -1.0 9.53380658669839e-06 -1.0 1.078687446476881e-14 -1.0 3.0571050855707084e-06 -1.0 0.0008231482263927564 -1.0 0.0038811025614225057 -1.0 0.0012297303081805393 -1.0 4.6842350882367666e-05 -1.0 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0.0002792896409558745 -1.0 0.01721806295736377 -1.0 0.11891251244906934 -1.0 0.019040707071165303 -1.0 0.00017329528628152923 -1.0 4.670673837764874e-08 -1.0 0.00012637534695610975 -1.0 0.00027052054298385255 -1.0 0.0001497958445994315 -1.0 3.2478301680021854e-08 -1.0 7.239087311622819e-08 -1.0 0.00046693784586018913 -1.0 0.001537590300149361 -1.0 0.00023686818807101741 -1.0 8.616027294555462e-07 -1.0 0.0002362355168421952 -1.0 0.05708498338352203 -1.0 0.42372713683725016 -1.0 0.054902816619122045 -1.0 0.00030002326388383244 -1.0 0.0023295401596005313 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 0.0027847329307350423 -1.0 0.003432919201293737 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 0.002672923773363389 -1.0 0.0003182108781955478 -1.0 0.05639071613735579 -1.0 0.4346984808472855 -1.0 0.05500108683629957 -1.0 0.00024175998138228048 -1.0 2.774067049392004e-07 -1.0 0.0003257646134837451 -1.0 0.0012648333883989995 -1.0 0.0002981891705231209 -1.0 4.986698316296507e-06 -1.0 3.419181071650816e-06 -1.0 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0.0001399807456616496 -1.0 4.279460329782799e-09 -1.0 1.2907171695294846e-13 -1.0 1.9257248009114363e-05 -1.0 9.332371598523186e-05 -1.0 1.3106335518133802e-05 -1.0 6.574706543646946e-16 -1.0 -1.0 -1.0 3.8270536025799805e-15 -1.0 2.101790478097517e-09 -1.0 9.904481358675478e-13 -1.0 -1.0</lower_ww_bounds>
+    <upper_ww_bounds>-10.0 -10.0 -10.0 5.447493368067179e-09 -10.0 4.0404626147344005e-12 -10.0 9.382800796426367e-16 -10.0 -10.0 -10.0 5.752120832330886e-10 -10.0 0.0003520674728719278 -10.0 0.0008760419353035269 -10.0 0.0003136829920007009 -10.0 -10.0 -10.0 4.233410073487772e-05 -10.0 0.0025519632243117644 -10.0 0.004777967757842694 -10.0 0.0035983547999740664 -10.0 4.625859206051786e-06 -10.0 4.587143797469485e-07 -10.0 0.0012918433677593976 -10.0 0.0046958956846326495 -10.0 0.0008862760180332873 -10.0 3.279091474401153e-05 -10.0 1.151457911727229e-09 -10.0 0.00022950716170641464 -10.0 0.0006922962992428139 -10.0 0.00021606550732170693 -10.0 2.6178780551069492e-15 -10.0 -10.0 -10.0 -10.0 -10.0 1.722191123879782e-07 -10.0 -10.0 -10.0 -10.0 -10.0 2.123498061876322e-12 -10.0 0.00014628847959717226 -10.0 0.0002777791741571174 -10.0 9.53380658669839e-05 -10.0 1.078687446476881e-13 -10.0 3.0571050855707086e-05 -10.0 0.008231482263927564 -10.0 0.03881102561422506 -10.0 0.012297303081805393 -10.0 0.00046842350882367664 -10.0 0.0016757997590048828 -10.0 0.11724714376848187 -10.0 0.6827568359008944 -10.0 0.11440381012864086 -10.0 0.003782322077390033 -10.0 0.0005098747893974248 -10.0 0.1208261392046878 -10.0 0.6428782790571179 -10.0 0.11294708566804167 -10.0 0.0008168081568291806 -10.0 0.000234750482787598 -10.0 0.008787411098754914 -10.0 0.0515132921322595 -10.0 0.011028020390507681 -10.0 1.0882011018684853e-05 -10.0 1.0588257623722672e-06 -10.0 0.00027044817176109554 -10.0 0.000262112392467961 -10.0 2.0707648529746893e-05 -10.0 -10.0 -10.0 1.3806577785135176e-05 -10.0 0.0010077009726691265 -10.0 0.004313097771132342 -10.0 0.001335621430311088 -10.0 3.792209399652396e-05 -10.0 0.0022057970761624767 -10.0 0.1884234453311008 -10.0 1.1510120013926417 -10.0 0.1825795827020865 -10.0 0.0020848309220162036 -10.0 0.013332554437480326 -10.0 4.931708164307665 -10.0 -10.0 -10.0 4.899952354672477 -10.0 0.014957033281168012 -10.0 0.01550036616828061 -10.0 4.897735047310072 -10.0 -10.0 -10.0 5.0 -10.0 0.011076796092102187 -10.0 0.002792896409558745 -10.0 0.17218062957363772 -10.0 1.1891251244906933 -10.0 0.19040707071165303 -10.0 0.0017329528628152924 -10.0 4.670673837764874e-07 -10.0 0.0012637534695610975 -10.0 0.0027052054298385255 -10.0 0.001497958445994315 -10.0 3.2478301680021856e-07 -10.0 7.239087311622819e-07 -10.0 0.004669378458601891 -10.0 0.01537590300149361 -10.0 0.002368681880710174 -10.0 8.61602729455546e-06 -10.0 0.002362355168421952 -10.0 0.5708498338352204 -10.0 4.237271368372502 -10.0 0.5490281661912204 -10.0 0.0030002326388383245 -10.0 0.023295401596005315 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 0.027847329307350423 -10.0 0.03432919201293737 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 0.026729237733633893 -10.0 0.0031821087819554777 -10.0 0.5639071613735579 -10.0 4.346984808472855 -10.0 0.5500108683629957 -10.0 0.0024175998138228046 -10.0 2.774067049392004e-06 -10.0 0.0032576461348374514 -10.0 0.012648333883989995 -10.0 0.002981891705231209 -10.0 4.986698316296507e-05 -10.0 3.419181071650816e-05 -10.0 0.0010814452764860557 -10.0 0.006934466049804711 -10.0 0.0025106741008066318 -10.0 5.515522196186723e-05 -10.0 0.0014242053195529865 -10.0 0.18541112907970003 -10.0 1.2364069326350118 -10.0 0.17602971074583987 -10.0 0.0021305826462238594 -10.0 0.015016500045374082 -10.0 4.903495510314233 -10.0 -10.0 -10.0 4.952432605379829 -10.0 0.01769327994909422 -10.0 0.00930449789620512 -10.0 4.873897581604287 -10.0 -10.0 -10.0 4.981497862127628 -10.0 0.011273340972061077 -10.0 0.0014126153603265096 -10.0 0.1729816946709781 -10.0 1.1819165666519222 -10.0 0.1780262484403569 -10.0 0.001319677450153934 -10.0 2.210707325798542e-06 -10.0 0.0017287873387910357 -10.0 0.0051970818427099184 -10.0 0.0009256616392487858 -10.0 1.0325207105027347e-07 -10.0 2.027381220364608e-15 -10.0 0.00012257360246712473 -10.0 0.00038561696337086434 -10.0 0.00018064550990294753 -10.0 7.6915277818290365e-16 -10.0 0.00012980629616179268 -10.0 0.01178616843705956 -10.0 0.04441328419270369 -10.0 0.008919466185572301 -10.0 5.272021975060514e-05 -10.0 0.0016079768514218546 -10.0 0.11128424196387618 -10.0 0.6433819434431045 -10.0 0.10955637485319249 -10.0 0.0008370896020975689 -10.0 0.0022234471595443312 -10.0 0.1102706140784316 -10.0 0.6684484191345574 -10.0 0.11113024641218336 -10.0 0.0012558879474108074 -10.0 0.00010044390728858278 -10.0 0.00808100901275472 -10.0 0.05384625561469684 -10.0 0.01146714816952061 -10.0 3.574060778841494e-05 -10.0 2.2512327083641864e-09 -10.0 0.0006717625881826168 -10.0 0.0006949587369646498 -10.0 0.0002909334597507523 -10.0 -10.0 -10.0 -10.0 -10.0 8.582011716859219e-14 -10.0 8.304845400451923e-07 -10.0 4.5757238372058235e-10 -10.0 -10.0 -10.0 1.3116807133030034e-14 -10.0 0.00026866894852481903 -10.0 0.00043166052799897375 -10.0 6.401055165502385e-05 -10.0 5.726152701849347e-15 -10.0 8.541606727879512e-06 -10.0 0.0020293965453828133 -10.0 0.004299195570308035 -10.0 0.0008544159282151438 -10.0 1.0995322456277382e-05 -10.0 1.289814511028015e-05 -10.0 0.0011042047857307254 -10.0 0.006870228272606287 -10.0 0.0013998074566164962 -10.0 4.279460329782799e-08 -10.0 1.2907171695294846e-12 -10.0 0.00019257248009114364 -10.0 0.0009332371598523186 -10.0 0.000131063355181338 -10.0 6.574706543646946e-15 -10.0 -10.0 -10.0 3.8270536025799805e-14 -10.0 2.101790478097517e-08 -10.0 9.904481358675478e-12 -10.0 -10.0</upper_ww_bounds>
+    <survival_ratio>3</survival_ratio>
+    <max_split>10</max_split>
+    <weight_cutoff>1e-38</weight_cutoff>
+  </weight_windows>
+  <max_splits>200</max_splits>
+</settings>
+<?xml version='1.0' encoding='utf-8'?>
+<tallies>
+  <mesh id="1">
+    <dimension>5 10 15</dimension>
+    <lower_left>-240 -240 -240</lower_left>
+    <upper_right>240 240 240</upper_right>
+  </mesh>
+  <filter id="1" type="mesh">
+    <bins>1</bins>
+  </filter>
+  <filter id="2" type="energy">
+    <bins>0.0 0.5 20000000.0</bins>
+  </filter>
+  <filter id="3" type="particle">
+    <bins>neutron photon</bins>
+  </filter>
+  <tally id="1">
+    <filters>1 2 3</filters>
+    <scores>flux</scores>
+  </tally>
+</tallies>
diff --git a/tests/regression_tests/weightwindows/results_true.dat b/tests/regression_tests/weightwindows/results_true.dat
new file mode 100644
index 000000000..5e8a41faf
--- /dev/null
+++ b/tests/regression_tests/weightwindows/results_true.dat
@@ -0,0 +1 @@
+c77af605fb23bc3f03bb304b82cc2d0d9efe7826167f43202fa0ecba17e6a77fc1a91bf9f4b23f175ae98c04c69067bffb96d7d68e30a126f5fa7c0080ea8367
\ No newline at end of file
diff --git a/tests/regression_tests/weightwindows/test.py b/tests/regression_tests/weightwindows/test.py
new file mode 100644
index 000000000..63d773cbc
--- /dev/null
+++ b/tests/regression_tests/weightwindows/test.py
@@ -0,0 +1,110 @@
+from copy import deepcopy
+import pytest
+import numpy as np
+
+import openmc
+from openmc.stats import Discrete, Point
+
+from tests.testing_harness import HashedPyAPITestHarness
+
+
+@pytest.fixture
+def model():
+    model = openmc.Model()
+
+    # materials (M4 steel alloy)
+    m4 = openmc.Material()
+    m4.set_density('g/cc', 2.3)
+    m4.add_nuclide('H1', 0.168018676)
+    m4.add_nuclide("H2", 1.93244e-05)
+    m4.add_nuclide("O16", 0.561814465)
+    m4.add_nuclide("O17", 0.00021401)
+    m4.add_nuclide("Na23", 0.021365)
+    m4.add_nuclide("Al27", 0.021343)
+    m4.add_nuclide("Si28", 0.187439342)
+    m4.add_nuclide("Si29", 0.009517714)
+    m4.add_nuclide("Si30", 0.006273944)
+    m4.add_nuclide("Ca40", 0.018026179)
+    m4.add_nuclide("Ca42", 0.00012031)
+    m4.add_nuclide("Ca43", 2.51033e-05)
+    m4.add_nuclide("Ca44", 0.000387892)
+    m4.add_nuclide("Ca46", 7.438e-07)
+    m4.add_nuclide("Ca48", 3.47727e-05)
+    m4.add_nuclide("Fe54", 0.000248179)
+    m4.add_nuclide("Fe56", 0.003895875)
+    m4.add_nuclide("Fe57", 8.99727e-05)
+    m4.add_nuclide("Fe58", 1.19737e-05)
+
+    s0 = openmc.Sphere(r=240)
+    s1 = openmc.Sphere(r=250, boundary_type='vacuum')
+
+    c0 = openmc.Cell(fill=m4, region=-s0)
+    c1 = openmc.Cell(region=+s0 & -s1)
+
+    model.geometry = openmc.Geometry([c0, c1])
+
+    # settings
+    settings = model.settings
+    settings.run_mode = 'fixed source'
+    settings.particles = 200
+    settings.batches = 2
+    settings.max_splits = 200
+    settings.photon_transport = True
+    space = Point((0.001, 0.001, 0.001))
+    energy = Discrete([14E6], [1.0])
+
+    settings.source = openmc.Source(space=space, energy=energy)
+
+    # tally
+    mesh = openmc.RegularMesh()
+    mesh.lower_left = (-240, -240, -240)
+    mesh.upper_right = (240, 240, 240)
+    mesh.dimension = (5, 10, 15)
+
+    mesh_filter = openmc.MeshFilter(mesh)
+
+    e_bnds = [0.0, 0.5, 2E7]
+    energy_filter = openmc.EnergyFilter(e_bnds)
+
+    particle_filter = openmc.ParticleFilter(['neutron', 'photon'])
+
+    tally = openmc.Tally()
+    tally.filters = [mesh_filter, energy_filter, particle_filter]
+    tally.scores = ['flux']
+
+    model.tallies.append(tally)
+
+    # weight windows
+
+    # load pre-generated weight windows
+    # (created using the same tally as above)
+    ww_n_lower_bnds = np.loadtxt('ww_n.txt')
+    ww_p_lower_bnds = np.loadtxt('ww_p.txt')
+
+    # create a mesh matching the one used
+    # to generate the weight windows
+    ww_mesh = openmc.RegularMesh()
+    ww_mesh.lower_left = (-240, -240, -240)
+    ww_mesh.upper_right = (240, 240, 240)
+    ww_mesh.dimension = (5, 6, 7)
+
+    ww_n = openmc.WeightWindows(ww_mesh,
+                                ww_n_lower_bnds,
+                                None,
+                                10.0,
+                                e_bnds)
+
+    ww_p = openmc.WeightWindows(ww_mesh,
+                                ww_p_lower_bnds,
+                                None,
+                                10.0,
+                                e_bnds)
+
+    model.settings.weight_windows = [ww_n, ww_p]
+
+    return model
+
+
+def test_weightwindows(model):
+    test = HashedPyAPITestHarness('statepoint.2.h5', model)
+    test.main()
diff --git a/tests/regression_tests/weightwindows/ww_n.txt b/tests/regression_tests/weightwindows/ww_n.txt
new file mode 100644
index 000000000..dbb49537b
--- /dev/null
+++ b/tests/regression_tests/weightwindows/ww_n.txt
@@ -0,0 +1,420 @@
+-1.000000000000000000e+00
+-1.000000000000000000e+00
+-1.000000000000000000e+00
+-1.000000000000000000e+00
+-1.000000000000000000e+00
+-1.000000000000000000e+00
+-1.000000000000000000e+00
+-1.000000000000000000e+00
+-1.000000000000000000e+00
+-1.000000000000000000e+00
+-1.000000000000000000e+00
+-1.000000000000000000e+00
+7.695031155172815952e-14
+-1.000000000000000000e+00
+7.476545089278482294e-06
+-1.000000000000000000e+00
+2.161215042522170282e-06
+-1.000000000000000000e+00
+-1.000000000000000000e+00
+-1.000000000000000000e+00
+2.136040178434497494e-18
+-1.000000000000000000e+00
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diff --git a/tests/regression_tests/weightwindows/ww_p.txt b/tests/regression_tests/weightwindows/ww_p.txt
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diff --git a/tests/unit_tests/weightwindows/__init__.py b/tests/unit_tests/weightwindows/__init__.py
new file mode 100644
index 000000000..e69de29bb
diff --git a/tests/unit_tests/weightwindows/test.py b/tests/unit_tests/weightwindows/test.py
new file mode 100644
index 000000000..aaa0bb523
--- /dev/null
+++ b/tests/unit_tests/weightwindows/test.py
@@ -0,0 +1,194 @@
+import os
+from pathlib import Path
+
+import numpy as np
+import pytest
+from uncertainties import ufloat
+
+import openmc
+from openmc.stats import Discrete, Point
+
+from tests import cdtemp
+
+@pytest.fixture
+def model():
+    openmc.reset_auto_ids()
+    model = openmc.Model()
+
+    # materials (M4 steel alloy)
+    m4 = openmc.Material()
+    m4.set_density('g/cc', 2.3)
+    m4.add_nuclide('H1', 0.168018676)
+    m4.add_nuclide("H2", 1.93244e-05)
+    m4.add_nuclide("O16", 0.561814465)
+    m4.add_nuclide("O17", 0.00021401)
+    m4.add_nuclide("Na23", 0.021365)
+    m4.add_nuclide("Al27", 0.021343)
+    m4.add_nuclide("Si28", 0.187439342)
+    m4.add_nuclide("Si29", 0.009517714)
+    m4.add_nuclide("Si30", 0.006273944)
+    m4.add_nuclide("Ca40", 0.018026179)
+    m4.add_nuclide("Ca42", 0.00012031)
+    m4.add_nuclide("Ca43", 2.51033e-05)
+    m4.add_nuclide("Ca44", 0.000387892)
+    m4.add_nuclide("Ca46", 7.438e-07)
+    m4.add_nuclide("Ca48", 3.47727e-05)
+    m4.add_nuclide("Fe54", 0.000248179)
+    m4.add_nuclide("Fe56", 0.003895875)
+    m4.add_nuclide("Fe57", 8.99727e-05)
+    m4.add_nuclide("Fe58", 1.19737e-05)
+
+    s0 = openmc.Sphere(r=240)
+    s1 = openmc.Sphere(r=250, boundary_type='vacuum')
+
+    c0 = openmc.Cell(fill=m4, region=-s0)
+    c1 = openmc.Cell(region=+s0 & -s1)
+
+    model.geometry = openmc.Geometry([c0, c1])
+
+    # settings
+    settings = model.settings
+    settings.run_mode = 'fixed source'
+    settings.particles = 500
+    settings.batches = 2
+    settings.max_splits = 100
+    settings.photon_transport = True
+    space = Point((0.001, 0.001, 0.001))
+    energy = Discrete([14E6], [1.0])
+
+    settings.source = openmc.Source(space=space, energy=energy)
+
+    # tally
+    mesh = openmc.RegularMesh()
+    mesh.lower_left = (-240, -240, -240)
+    mesh.upper_right = (240, 240, 240)
+    mesh.dimension = (3, 5, 7)
+
+    mesh_filter = openmc.MeshFilter(mesh)
+
+    e_bnds = [0.0, 0.5, 2E7]
+    energy_filter = openmc.EnergyFilter(e_bnds)
+
+    particle_filter = openmc.ParticleFilter(['neutron', 'photon'])
+
+    tally = openmc.Tally()
+    tally.filters = [mesh_filter, energy_filter, particle_filter]
+    tally.scores = ['flux']
+
+    model.tallies.append(tally)
+
+    return model
+
+
+def test_weightwindows(model):
+
+    ww_files = ('ww_n.txt', 'ww_p.txt')
+    cwd = Path(__file__).parent.absolute()
+    filepaths = [cwd / Path(f) for f in ww_files]
+
+    with cdtemp(filepaths):
+        # run once with variance reduction off
+        model.settings.weight_windows_on = False
+        analog_sp = model.run()
+        os.rename(analog_sp, 'statepoint.analog.h5')
+
+        # weight windows
+
+        # load pre-generated weight windows
+        # (created using the same tally as above)
+        ww_n_lower_bnds = np.loadtxt('ww_n.txt')
+        ww_p_lower_bnds = np.loadtxt('ww_p.txt')
+
+        # create a mesh matching the one used
+        # to generate the weight windows
+        ww_mesh = openmc.RegularMesh()
+        ww_mesh.lower_left = (-240, -240, -240)
+        ww_mesh.upper_right = (240, 240, 240)
+        ww_mesh.dimension = (5, 6, 7)
+
+        # energy bounds matching those of the
+        # generated weight windows
+        e_bnds = [0.0, 0.5, 2E7]
+
+        ww_n = openmc.WeightWindows(ww_mesh,
+                                    ww_n_lower_bnds,
+                                    None,
+                                    10.0,
+                                    e_bnds,
+                                    survival_ratio=1.01)
+
+        ww_p = openmc.WeightWindows(ww_mesh,
+                                    ww_p_lower_bnds,
+                                    None,
+                                    10.0,
+                                    e_bnds,
+                                    survival_ratio=1.01)
+
+        model.settings.weight_windows = [ww_n, ww_p]
+
+
+        # run again with variance reduction on
+        model.settings.weight_windows_on = True
+        ww_sp = model.run()
+        os.rename(ww_sp, 'statepoint.ww.h5')
+
+        # load both statepoints and examine results
+        asp = openmc.StatePoint('statepoint.analog.h5')
+        wsp = openmc.StatePoint('statepoint.ww.h5')
+
+        analog_tally = asp.tallies[1]
+        ww_tally = wsp.tallies[1]
+
+        def compare_results(particle, analog_tally, ww_tally):
+            # get values from each of the tallies
+            an_mean = analog_tally.get_values(filters=[openmc.ParticleFilter],
+                                              filter_bins=[(particle,)])
+            ww_mean = ww_tally.get_values(filters=[openmc.ParticleFilter],
+                                          filter_bins=[(particle,)])
+
+            # expect that more bins were scored with weight windows than
+            # the analog run
+            assert np.count_nonzero(an_mean) < np.count_nonzero(ww_mean)
+
+            an_rel_err = analog_tally.get_values(filters=[openmc.ParticleFilter],
+                                                 filter_bins=[(particle,)],
+                                                 value='rel_err')
+            ww_rel_err = ww_tally.get_values(filters=[openmc.ParticleFilter],
+                                             filter_bins=[(particle,)],
+                                             value='rel_err')
+
+            an_rel_err[an_mean == 0.0] = 1.0
+            ww_rel_err[ww_mean == 0.0] = 1.0
+
+            an_avg_rel_err = np.mean(an_rel_err)
+            ww_avg_rel_err = np.mean(ww_rel_err)
+
+            # expect that the average relative error in the tally
+            # decreases
+            assert an_avg_rel_err > ww_avg_rel_err
+
+            # ensure that the value of the mesh bin containing the
+            # source is statistically similar in both runs
+            an_std_dev = analog_tally.get_values(filters=[openmc.ParticleFilter],
+                                                 filter_bins=[(particle,)],
+                                                 value='std_dev')
+            ww_std_dev = ww_tally.get_values(filters=[openmc.ParticleFilter],
+                                             filter_bins=[(particle,)],
+                                             value='std_dev')
+
+            # index of the mesh bin containing the source for the higher
+            # energy group
+            source_bin_idx = (an_mean.shape[0]//2, 0, 0)
+
+            an_source_bin = ufloat(an_mean[source_bin_idx],
+                                   an_std_dev[source_bin_idx])
+            ww_source_bin = ufloat(ww_mean[source_bin_idx],
+                                   ww_std_dev[source_bin_idx])
+
+            diff = an_source_bin - ww_source_bin
+
+            # check that values are within two combined standard deviations
+            assert abs(diff.nominal_value) / diff.std_dev < 2.0
+
+        compare_results('neutron', analog_tally, ww_tally)
+        compare_results('photon', analog_tally, ww_tally)
diff --git a/tests/unit_tests/weightwindows/ww_n.txt b/tests/unit_tests/weightwindows/ww_n.txt
new file mode 120000
index 000000000..3bd70d2c9
--- /dev/null
+++ b/tests/unit_tests/weightwindows/ww_n.txt
@@ -0,0 +1 @@
+../../regression_tests/weightwindows/ww_n.txt
\ No newline at end of file
diff --git a/tests/unit_tests/weightwindows/ww_p.txt b/tests/unit_tests/weightwindows/ww_p.txt
new file mode 120000
index 000000000..8a94de709
--- /dev/null
+++ b/tests/unit_tests/weightwindows/ww_p.txt
@@ -0,0 +1 @@
+../../regression_tests/weightwindows/ww_p.txt
\ No newline at end of file