diff --git a/docs/source/devguide/structures.rst b/docs/source/devguide/structures.rst
index aa22c40fe0..15777f6065 100644
--- a/docs/source/devguide/structures.rst
+++ b/docs/source/devguide/structures.rst
@@ -80,6 +80,71 @@ low-energy scattering, a :math:`S(\alpha,\beta)` can be associated with that
 material through the ``sab_table`` component. Again, this component contains the
 index in the ``sab_tables`` array from the `global module`_.
 
+-------
+Nuclide
+-------
+
+The Nuclide derived type stores cross section and interaction data for a nucleus
+and is defined in the `ace_header module`_. The ``energy`` component is an array
+that gives the discrete energies at which microscopic cross sections are
+tabulated. The actual microscopic cross sections are stored in a separate
+derived type, Reaction. An arrays of Reactions is present in the ``reactions``
+component. There are a few summary microscopic cross sections stored in other
+components, such as ``total``, ``elastic``, ``fission``, and ``nu_fission``.
+
+If a Nuclide is fissionable, the prompt and delayed neutron yield and energy
+distributions are also stored on the Nuclide type. Many nuclides also have
+unresolved resonance probability table data. If present, this data is stored in
+the component ``urr_data`` of derived type UrrData. A complete description of
+the probability table method is given in :ref:`probability_tables`.
+
+The list of nuclides present in a problem is stored in the ``nuclides`` array
+defined in the `global module`_.
+
+----------
+SAlphaBeta
+----------
+
+The SAlphaBeta derived type stores :math:`S(\alpha,\beta)` data to account for
+molecular binding effects when treating thermal scattering. Each SAlphaBeta
+table is associated with a specific nuclide as identified in the ``zaid``
+component. A complete description of the :math:`S(\alpha,\beta)` treatment can
+be found in :ref:`sab_tables`.
+
+---------
+XsListing
+---------
+
+The XsListing derived type stores information on the location of an ACE cross
+section table based on the data in cross_sections.xml and is defined in the
+`ace_header module`_. For each ``<ace_table>`` you see in cross_sections.xml,
+there is a XsListing with its information. When the user input is read, the
+array ``xs_listings`` in the `global module`_ that is of derived type XsListing
+is used to locate the ACE data to parse.
+
+--------------
+NuclideMicroXS
+--------------
+
+The NuclideMicroXS derived type, defined in the `ace_header module`_, acts as a
+'cache' for microscopic cross sections. As a particle is traveling through
+different materials, cross sections can be reused if the energy of the particle
+hasn't changed. The components ``total``, ``elastic``, ``absorption``,
+``fission``, and ``nu_fission`` represent those microscopic cross sections at
+the current energy of the particle for a given nuclide. An array ``micro_xs`` in
+the `global module`_ that is the same length as the ``nuclides`` array stores
+these cached cross sections for each nuclide in the problem.
+
+---------------
+MaterialMacroXS
+---------------
+
+In addition to the NuclideMicroXS type, there is also a MaterialMacroXS derived
+type, defined in the `ace_header module`_ that stored cached *macroscopic* cross
+sections for the current material. These macroscopic cross sections are used for
+both physics and tallying purposes. The variable ``material_xs`` in the `global
+module`_ is of type MaterialMacroXS.
+
 
 .. _derived types: http://nf.nci.org.au/training/FortranAdvanced/slides/slides.025.html
 .. _global module: https://github.com/mit-crpg/openmc/blob/master/src/global.F90
@@ -87,3 +152,4 @@ index in the ``sab_tables`` array from the `global module`_.
 .. _geometry_header module: https://github.com/mit-crpg/openmc/blob/master/src/geometry_header.F90
 .. _constants module: https://github.com/mit-crpg/openmc/blob/master/src/constants.F90
 .. _material_header module: https://github.com/mit-crpg/openmc/blob/master/src/material_header.F90
+.. _ace_header module: https://github.com/mit-crpg/openmc/blob/master/src/ace_header.F90
diff --git a/docs/source/methods/physics.rst b/docs/source/methods/physics.rst
index 865329fee2..0ed40130dd 100644
--- a/docs/source/methods/physics.rst
+++ b/docs/source/methods/physics.rst
@@ -1272,6 +1272,8 @@ If is not accepted, then we repeat the process and resample a target speed and
 cosine until a combination is found that satisfies equation
 :eq:`freegas-accept-2`.
 
+.. _sab_tables:
+
 ------------
 |sab| Tables
 ------------
@@ -1419,7 +1421,9 @@ uniformly and then the final cosine is interpolated on the incoming energy grid:
     \mu = \mu_{i,j,k} + f (\mu_{i+1,j,k} - \mu_{i,j,k})
 
 where :math:`\mu_{i,j,k}` is the k-th outgoing cosine corresponding to the j-th
-outgoing energy and the i-th incoming energy. 
+outgoing energy and the i-th incoming energy.
+
+.. _probability_tables:
 
 ----------------------------------------------
 Unresolved Resonance Region Probability Tables