diff --git a/include/openmc/distribution.h b/include/openmc/distribution.h
index b8ebcabf4..6db3c1618 100644
--- a/include/openmc/distribution.h
+++ b/include/openmc/distribution.h
@@ -37,6 +37,7 @@ public:
   //! \return Sampled value
   double sample() const;
 
+  // Properties
   const std::vector<double>& x() const { return x_; }
   const std::vector<double>& p() const { return p_; }
 private:
diff --git a/include/openmc/distribution_spatial.h b/include/openmc/distribution_spatial.h
index e94005f0f..248f03588 100644
--- a/include/openmc/distribution_spatial.h
+++ b/include/openmc/distribution_spatial.h
@@ -48,6 +48,8 @@ public:
   //! Sample a position from the distribution
   //! \return Sampled position
   Position sample() const;
+
+  // Properties
   bool only_fissionable() const { return only_fissionable_; }
 private:
   Position lower_left_; //!< Lower-left coordinates of box
diff --git a/include/openmc/file_utils.h b/include/openmc/file_utils.h
index 0a24cb547..f9c23468d 100644
--- a/include/openmc/file_utils.h
+++ b/include/openmc/file_utils.h
@@ -6,12 +6,15 @@
 
 namespace openmc {
 
+//! Determine if a file exists
+//! \param[in] filename Path to file
+//! \return Whether file exists
 inline bool file_exists(const std::string& filename)
 {
   std::ifstream s {filename};
   return s.good();
 }
 
-}
+} // namespace openmc
 
-#endif // OPENMC_FILE_UTILS_H
\ No newline at end of file
+#endif // OPENMC_FILE_UTILS_H
diff --git a/include/openmc/nuclide.h b/include/openmc/nuclide.h
index 8a19791ba..e3d43450d 100644
--- a/include/openmc/nuclide.h
+++ b/include/openmc/nuclide.h
@@ -1,3 +1,6 @@
+//! \file nuclide.h
+//! \brief Nuclide type and other associated types/data
+
 #ifndef OPENMC_NUCLIDE_H
 #define OPENMC_NUCLIDE_H
 
@@ -7,6 +10,12 @@
 
 namespace openmc {
 
+//==============================================================================
+// Global variables
+//==============================================================================
+
+// Minimum/maximum transport energy for each particle type. Order corresponds to
+// that of the ParticleType enum
 extern std::array<double, 2> energy_min;
 extern std::array<double, 2> energy_max;
 
diff --git a/include/openmc/simulation.h b/include/openmc/simulation.h
index a1350c936..daf7393fb 100644
--- a/include/openmc/simulation.h
+++ b/include/openmc/simulation.h
@@ -1,3 +1,6 @@
+//! \file simulation.h
+//! \brief Variables/functions related to a running simulation
+
 #ifndef OPENMC_SIMULATION_H
 #define OPENMC_SIMULATION_H
 
@@ -6,6 +9,10 @@
 
 namespace openmc {
 
+//==============================================================================
+// Global variables
+//==============================================================================
+
 extern "C" int openmc_current_batch;
 extern "C" int openmc_current_gen;
 extern "C" int64_t openmc_current_work;
@@ -16,7 +23,14 @@ extern std::vector<int64_t> work_index;
 
 #pragma omp threadprivate(openmc_current_work)
 
+//==============================================================================
+// Functions
+//==============================================================================
+
+//! Initialize simulation
 extern "C" void openmc_simulation_init_c();
+
+//! Determine number of particles to transport per process
 void calculate_work();
 
 } // namespace openmc
diff --git a/include/openmc/source.h b/include/openmc/source.h
index ac5f69a2f..2e8c9491a 100644
--- a/include/openmc/source.h
+++ b/include/openmc/source.h
@@ -1,3 +1,6 @@
+//! \file source.h
+//! \brief External source distributions
+
 #ifndef OPENMC_SOURCE_H
 #define OPENMC_SOURCE_H
 
@@ -19,17 +22,22 @@ namespace openmc {
 
 class SourceDistribution {
 public:
+  // Constructors
   SourceDistribution(UPtrSpace space, UPtrAngle angle, UPtrDist energy);
   explicit SourceDistribution(pugi::xml_node node);
 
+  //! Sample from the external source distribution
+  //! \return Sampled site
   Bank sample() const;
+
+  // Properties
   double strength() const { return strength_; }
 private:
-  ParticleType particle_ {ParticleType::neutron};
-  double strength_ {1.0};
-  UPtrSpace space_;
-  UPtrAngle angle_;
-  UPtrDist energy_;
+  ParticleType particle_ {ParticleType::neutron}; //!< Type of particle emitted
+  double strength_ {1.0}; //!< Source strength
+  UPtrSpace space_; //!< Spatial distribution
+  UPtrAngle angle_; //!< Angular distribution
+  UPtrDist energy_; //!< Energy distribution
 };
 
 //==============================================================================
@@ -47,6 +55,7 @@ extern "C" void initialize_source();
 
 //! Sample a site from all external source distributions in proportion to their
 //! source strength
+//! \return Sampled source site
 extern "C" Bank sample_external_source();
 
 } // namespace openmc
diff --git a/src/nuclide.cpp b/src/nuclide.cpp
index f7f5399ba..79e2289ae 100644
--- a/src/nuclide.cpp
+++ b/src/nuclide.cpp
@@ -2,10 +2,17 @@
 
 namespace openmc {
 
-// Order corresponds to ParticleType enum
+//==============================================================================
+// Global variables
+//==============================================================================
+
 std::array<double, 2> energy_min {0.0, 0.0};
 std::array<double, 2> energy_max {INFTY, INFTY};
 
+//==============================================================================
+// Fortran compatibility functions
+//==============================================================================
+
 extern "C" void
 set_particle_energy_bounds(int particle, double E_min, double E_max)
 {
diff --git a/src/simulation.cpp b/src/simulation.cpp
index 19cc78d19..3514f4a47 100644
--- a/src/simulation.cpp
+++ b/src/simulation.cpp
@@ -22,8 +22,22 @@ int openmc_run() {
 
 namespace openmc {
 
+//==============================================================================
+// Global variables
+//==============================================================================
+
 std::vector<int64_t> work_index;
 
+//==============================================================================
+// Functions
+//==============================================================================
+
+void openmc_simulation_init_c()
+{
+  // Determine how much work each process should do
+  calculate_work();
+}
+
 void calculate_work()
 {
   // Determine minimum amount of particles to simulate on each processor
@@ -48,10 +62,4 @@ void calculate_work()
   }
 }
 
-void openmc_simulation_init_c()
-{
-  // Determine how much work each process should do
-  calculate_work();
-}
-
 } // namespace openmc