diff --git a/CMakeLists.txt b/CMakeLists.txt
index 2496c559d..823899c26 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -296,6 +296,7 @@ if (CLAD_INCLUDE_DOCS)
 endif()
 
 if (NOT CLAD_BUILD_STATIC_ONLY)
+  add_subdirectory(unittests)
   add_subdirectory(test)
   add_subdirectory(demos/ErrorEstimation/CustomModel)
   add_subdirectory(demos/ErrorEstimation/PrintModel)
diff --git a/cmake/modules/AddCladBenchmark.cmake b/cmake/modules/AddCladBenchmark.cmake
index 17983d0aa..aae26a223 100644
--- a/cmake/modules/AddCladBenchmark.cmake
+++ b/cmake/modules/AddCladBenchmark.cmake
@@ -10,31 +10,88 @@ string(REPLACE "/" "" CURRENT_REPO_COMMIT ${CURRENT_REPO_COMMIT})
 set_property(DIRECTORY APPEND PROPERTY
              CMAKE_CONFIGURE_DEPENDS "${CMAKE_SOURCE_DIR}/.git/HEAD")
 
-
 # Change the default compiler to the clang which we run clad upon.
 set(CMAKE_CXX_COMPILER ${LLVM_TOOLS_BINARY_DIR}/clang)
 
-#----------------------------------------------------------------------------
-# function CB_ADD_GBENCHMARK(<benchmark> source1 source2... LIBRARIES libs)
-#----------------------------------------------------------------------------
-function(CB_ADD_GBENCHMARK benchmark)
-  cmake_parse_arguments(ARG "" "LABEL" "DEPENDS;LIBRARIES" ${ARGN})
-  set(source_files ${ARG_UNPARSED_ARGUMENTS})
-
-  add_executable(${benchmark} ${source_files})
+#-------------------------------------------------------------------------------
+# function ENABLE_CLAD_FOR_EXECUTABLE(<executable>
+#   DEPENDS dependencies...
+#     A list of targets that the executable depends on.
+#   LIBRARIES libraries...
+#     A list of libraries to be linked in. Defaults to stdc++ pthread m.
+# )
+#-------------------------------------------------------------------------------
+function(ENABLE_CLAD_FOR_EXECUTABLE executable)
+  if (NOT TARGET ${executable})
+    message(FATAL_ERROR "'${executable}' is not a valid target.")
+  endif()
 
   # Add the clad plugin
-  target_compile_options(${benchmark} PUBLIC -fplugin=$<TARGET_FILE:clad>)
+  target_compile_options(${executable} PUBLIC -fplugin=$<TARGET_FILE:clad>)
 
   # Debugging. Emitting the derivatives' source code.
-  #target_compile_options(${benchmark} PUBLIC "SHELL:-Xclang -plugin-arg-clad"
+  #target_compile_options(${executable} PUBLIC "SHELL:-Xclang -plugin-arg-clad"
   #  "SHELL: -Xclang -fdump-derived-fn")
 
   # Debugging. Emit llvm IR.
-  #target_compile_options(${benchmark} PUBLIC -S -emit-llvm)
+  #target_compile_options(${executable} PUBLIC -S -emit-llvm)
 
   # Debugging. Optimization misses.
-  #target_compile_options(${benchmark} PUBLIC "SHELL:-Xclang -Rpass-missed=.*inline.*")
+  #target_compile_options(${executable} PUBLIC "SHELL:-Xclang -Rpass-missed=.*inline.*")
+
+  # Clad requires us to link against these libraries.
+  target_link_libraries(${executable} PUBLIC stdc++ pthread m)
+
+  target_include_directories(${executable} PUBLIC ${CMAKE_CURRENT_BINARY_DIR})
+  set_property(TARGET ${executable} PROPERTY RUNTIME_OUTPUT_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR})
+
+  add_dependencies(${executable} clad)
+  # If clad.so changes we don't need to relink but to rebuild the source files.
+  # $<TARGET_FILE:clad> does not work for OBJECT_DEPENDS.
+  set (CLAD_SO_PATH "${CMAKE_LIBRARY_OUTPUT_DIRECTORY}/clad${CMAKE_SHARED_LIBRARY_SUFFIX}")
+  set_source_files_properties(${source_files} PROPERTY OBJECT_DEPENDS ${CLAD_SO_PATH})
+
+  # Add dependencies to executable
+  if(ARG_DEPENDS)
+    add_dependencies(${executable} ${ARG_DEPENDS})
+  endif(ARG_DEPENDS)
+
+
+endfunction(ENABLE_CLAD_FOR_EXECUTABLE)
+
+#-------------------------------------------------------------------------------
+# function ADD_CLAD_EXECUTABLE(<executable> sources...
+#   DEPENDS dependencies...
+#     A list of targets that the executable depends on.
+#   LIBRARIES libraries...
+#     A list of libraries to be linked in. Defaults to stdc++ pthread m.
+# )
+#-------------------------------------------------------------------------------
+function(ADD_CLAD_EXECUTABLE executable)
+  cmake_parse_arguments(ARG "" "DEPENDS;LIBRARIES" "" ${ARGN})
+
+  set(source_files ${ARG_UNPARSED_ARGUMENTS})
+
+  add_executable(${executable} ${source_files})
+
+  ENABLE_CLAD_FOR_EXECUTABLE(${executable} ${ARGN})
+
+endfunction(ADD_CLAD_EXECUTABLE)
+
+#-------------------------------------------------------------------------------
+# function CB_ADD_GBENCHMARK(<benchmark> sources
+#   LABEL <short|long>
+#     A label that classifies how much time a benchmark is expected to take.
+#     Short benchmarks time out at 1200 seconds, long at 2400.
+#   DEPENDS dependencies...
+#     A list of targets that the executable depends on.
+#   LIBRARIES libraries...
+#     A list of libraries to be linked in. Defaults to stdc++ pthread m.
+# )
+#-------------------------------------------------------------------------------
+function(CB_ADD_GBENCHMARK benchmark)
+  cmake_parse_arguments(ARG "" "LABEL" "" ${ARGN})
+  ADD_CLAD_EXECUTABLE(${benchmark} ${ARG_UNPARSED_ARGUMENTS})
 
   # Optimize the produced code.
   target_compile_options(${benchmark} PUBLIC -O3)
@@ -42,23 +99,14 @@ function(CB_ADD_GBENCHMARK benchmark)
   # Turn off numerical diff fallback.
   target_compile_definitions(${benchmark} PUBLIC CLAD_NO_NUM_DIFF)
 
-  # Clad requires us to link against these libraries.
-  target_link_libraries(${benchmark} PUBLIC stdc++ pthread m)
-
-  target_include_directories(${benchmark} PUBLIC ${CMAKE_CURRENT_BINARY_DIR} ${GBENCHMARK_INCLUDE_DIR})
-  set_property(TARGET ${benchmark} PROPERTY RUNTIME_OUTPUT_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR})
+  # Add GBenchmark includes
+  target_include_directories(${executable} PUBLIC ${GBENCHMARK_INCLUDE_DIR})
 
   target_link_libraries(${benchmark} PUBLIC ${ARG_LIBRARIES} gbenchmark)
   if (NOT APPLE)
     target_link_libraries(${benchmark} PUBLIC rt)
   endif()
 
-  add_dependencies(${benchmark} clad)
-  # If clad.so changes we don't need to relink but to rebuild the source files.
-  # $<TARGET_FILE:clad> does not work for OBJECT_DEPENDS.
-  set (CLAD_SO_PATH "${CMAKE_LIBRARY_OUTPUT_DIRECTORY}/clad${CMAKE_SHARED_LIBRARY_SUFFIX}")
-  set_source_files_properties(${source_files} PROPERTY OBJECT_DEPENDS ${CLAD_SO_PATH})
-
   set (TIMEOUT_VALUE 1200)
   set (LABEL "short")
   if (ARG_LABEL AND "${ARG_LABEL}" STREQUAL "long")
@@ -66,11 +114,6 @@ function(CB_ADD_GBENCHMARK benchmark)
     set (LABEL "long")
   endif()
 
-  # Add dependencies to benchmark
-  if(ARG_DEPENDS)
-    add_dependencies(${benchmark} ${ARG_DEPENDS})
-  endif()
-
   # Add benchmark as a CTest
   add_test(NAME clad-${benchmark}
     COMMAND ${benchmark} --benchmark_out_format=json
@@ -82,3 +125,27 @@ function(CB_ADD_GBENCHMARK benchmark)
                        RUN_SERIAL TRUE
                        DEPENDS ${benchmark})
 endfunction(CB_ADD_GBENCHMARK)
+
+#-------------------------------------------------------------------------------
+# function ADD_CLAD_GTEST(<gtest> sources
+#   DEPENDS dependencies...
+#     A list of targets that the executable depends on.
+#   LIBRARIES libraries...
+#     A list of libraries to be linked in. Defaults to stdc++ pthread m.
+# )
+#-------------------------------------------------------------------------------
+function(ADD_CLAD_GTEST gtest)
+  ADD_CLAD_EXECUTABLE(${gtest} ${ARGN})
+
+  set(gtest_libs gtest gtest_main)
+  # Clang prior than clang13 (I think) merges both gmock into gtest.
+  if (TARGET gmock)
+    list(APPEND gtest_libs gmock gmock_main)
+  endif()
+
+  target_link_libraries(${gtest} PUBLIC ${ARG_LIBRARIES} ${gtest_libs})
+  if (NOT APPLE)
+    #target_link_libraries(${gtest} PUBLIC rt)
+  endif()
+
+endfunction(ADD_CLAD_GTEST)
diff --git a/include/clad/Differentiator/CladConfig.h b/include/clad/Differentiator/CladConfig.h
index 8ed22421b..4ad838a31 100644
--- a/include/clad/Differentiator/CladConfig.h
+++ b/include/clad/Differentiator/CladConfig.h
@@ -54,16 +54,10 @@ constexpr unsigned GetBitmaskedOpts(unsigned const first, Opts... opts) {
 // Define trap function that is a CUDA compatible replacement for
 // exit(int code) function
 #ifdef __CUDACC__
-__device__ void trap(int code) {
-  asm("trap;");
-}
-__host__ void trap(int code) {
-  exit(code);
-}
+inline __device__ void trap(int code) { asm("trap;"); }
+inline __host__ void trap(int code) { exit(code); }
 #else
-void trap(int code) {
-  exit(code);
-}
+inline void trap(int code) { exit(code); }
 #endif
 
 #ifdef  __CUDACC__
diff --git a/include/clad/Differentiator/Differentiator.h b/include/clad/Differentiator/Differentiator.h
index 49797a7ba..5468d6773 100644
--- a/include/clad/Differentiator/Differentiator.h
+++ b/include/clad/Differentiator/Differentiator.h
@@ -27,20 +27,20 @@ template <typename T, typename U> struct ValueAndAdjoint {
 };
 
   /// \returns the size of a c-style string
-  CUDA_HOST_DEVICE unsigned int GetLength(const char* code) {
-    unsigned int count;
-    const char* code_copy = code;
-    #ifdef __CUDACC__
-      count = 0;
-      while (*code_copy != '\0') {
-        count++;
-        code_copy++;
-      }
-    #else
-      count = strlen(code_copy);
-    #endif
-    return count;
+inline CUDA_HOST_DEVICE unsigned int GetLength(const char* code) {
+  unsigned int count;
+  const char* code_copy = code;
+#ifdef __CUDACC__
+  count = 0;
+  while (*code_copy != '\0') {
+    count++;
+    code_copy++;
   }
+#else
+  count = strlen(code_copy);
+#endif
+    return count;
+}
 
   /// Tape type used for storing values in reverse-mode AD inside loops.
   template <typename T>
diff --git a/include/clad/Differentiator/NumericalDiff.h b/include/clad/Differentiator/NumericalDiff.h
index 5f090d793..e8ae560c0 100644
--- a/include/clad/Differentiator/NumericalDiff.h
+++ b/include/clad/Differentiator/NumericalDiff.h
@@ -73,7 +73,10 @@ namespace numerical_diff {
 
   /// A buffer manager to request for buffer space
   /// while forwarding reference/pointer args to the target function.
-  ManageBufferSpace bufferManager;
+  inline ManageBufferSpace& getBufferManager() {
+    static ManageBufferSpace bufMan;
+    return bufMan;
+  }
 
   /// The precision to do the numerical differentiation calculations in.
   using precision = double;
@@ -87,7 +90,7 @@ namespace numerical_diff {
   /// \param[in] \c h The h to make representable.
   ///
   /// \returns A value of h that does not result in catastrohic cancellation.
-  precision make_h_representable(precision x, precision h) {
+  inline precision make_h_representable(precision x, precision h) {
     precision xph = x + h;
     precision dx = xph - x;
 
@@ -104,7 +107,7 @@ namespace numerical_diff {
   /// \param[in] \c arg The input argument to adjust and get h for.
   ///
   /// \returns A calculated and adjusted h value.
-  precision get_h(precision arg) {
+  inline precision get_h(precision arg) {
     // First get a suitable h value, we do all of this in elevated precision
     // (default double). Maximum error in h = eps^4/5
     precision h = std::pow(11.25 * std::numeric_limits<precision>::epsilon(),
@@ -126,8 +129,8 @@ namespace numerical_diff {
   ///  belongs.
   /// \param[in] \c arrPos The position of the array element
   /// (-1 if parameter is scalar) to which the error belongs.
-  void printError(precision derivError, precision evalError, unsigned paramPos,
-                  int arrPos = -1) {
+  inline void printError(precision derivError, precision evalError,
+                         unsigned paramPos, int arrPos = -1) {
     if (arrPos != -1)
       printf("\nError Report for parameter at position %d and index %d:\n",
              paramPos, arrPos);
@@ -227,7 +230,7 @@ namespace numerical_diff {
     // this is required to make sure that we are retuning a deep copy
     // that is valid throughout the scope of the central_diff function.
     // Temp is system owned.
-    T* temp = bufferManager.make_buffer_space<T>(n);
+    T* temp = getBufferManager().make_buffer_space<T>(n);
     // deepcopy
     for (std::size_t j = 0; j < n; j++) {
       temp[j] = arg[j];
@@ -318,7 +321,7 @@ namespace numerical_diff {
 
         // five-point stencil formula = (4f[x+h, x-h] - f[x+2h, x-2h])/3
         _grad[i][j] = 4.0 * xf1 / 3.0 - xf2 / 3.0;
-        bufferManager.free_buffer();
+        getBufferManager().free_buffer();
       }
     }
   }
diff --git a/test/CMakeLists.txt b/test/CMakeLists.txt
index 823109517..a37a418ad 100644
--- a/test/CMakeLists.txt
+++ b/test/CMakeLists.txt
@@ -29,6 +29,13 @@ configure_lit_site_cfg(
   ${CMAKE_CURRENT_BINARY_DIR}/lit.site.cfg
   )
 
+configure_lit_site_cfg(
+  ${CMAKE_CURRENT_SOURCE_DIR}/Unit/lit.site.cfg.py.in
+  ${CMAKE_CURRENT_BINARY_DIR}/Unit/lit.site.cfg.py
+  MAIN_CONFIG
+  ${CMAKE_CURRENT_SOURCE_DIR}/Unit/lit.cfg.py
+  )
+
 option(CLANG_TEST_USE_VG "Run Clang tests under Valgrind" OFF)
 if(CLANG_TEST_USE_VG)
   set(CLANG_TEST_EXTRA_ARGS ${CLANG_TEST_EXTRA_ARGS} "--vg")
@@ -44,6 +51,12 @@ set(CLAD_TEST_PARAMS
   clad_site_config=${CMAKE_CURRENT_BINARY_DIR}/lit.site.cfg
   )
 
+# Unit tests
+list(APPEND CLAD_TEST_DEPS CladUnitTests)
+list(APPEND CLAD_TEST_PARAMS
+  clad_site_config=${CMAKE_CURRENT_BINARY_DIR}/Unit/lit.site.cfg
+  )
+
 add_custom_target(clad-test-depends DEPENDS ${CLAD_TEST_DEPS})
 set_target_properties(clad-test-depends PROPERTIES FOLDER "Clad tests")
 
diff --git a/unittests/Basic/CMakeLists.txt b/unittests/Basic/CMakeLists.txt
new file mode 100644
index 000000000..48da2fd0b
--- /dev/null
+++ b/unittests/Basic/CMakeLists.txt
@@ -0,0 +1,4 @@
+add_clad_unittest(DifferentiatorHTests
+  DifferentiatorH.cpp
+  MultiIncludeDifferentiatorH.cpp
+  )
diff --git a/unittests/Basic/DifferentiatorH.cpp b/unittests/Basic/DifferentiatorH.cpp
new file mode 100644
index 000000000..34157c910
--- /dev/null
+++ b/unittests/Basic/DifferentiatorH.cpp
@@ -0,0 +1,8 @@
+#include "clad/Differentiator/Differentiator.h"
+
+#include "gtest/gtest.h"
+
+TEST(DifferentiatorH, GetLength) {
+  EXPECT_TRUE(clad::GetLength("") == 0);
+  EXPECT_TRUE(clad::GetLength("abc") == 3);
+}
diff --git a/unittests/Basic/MultiIncludeDifferentiatorH.cpp b/unittests/Basic/MultiIncludeDifferentiatorH.cpp
new file mode 100644
index 000000000..d6e5c5179
--- /dev/null
+++ b/unittests/Basic/MultiIncludeDifferentiatorH.cpp
@@ -0,0 +1 @@
+#include "clad/Differentiator/Differentiator.h"
diff --git a/unittests/CMakeLists.txt b/unittests/CMakeLists.txt
new file mode 100644
index 000000000..5213b4611
--- /dev/null
+++ b/unittests/CMakeLists.txt
@@ -0,0 +1,35 @@
+add_custom_target(CladUnitTests)
+set_target_properties(CladUnitTests PROPERTIES FOLDER "Clang tests")
+
+# LLVM builds (not installed llvm) provides gtest.
+if (NOT TARGET gtest)
+  include(CladGoogleTest)
+  include(AddCladBenchmark)
+endif()
+
+# add_clad_unittest(test_dirname file1.cpp file2.cpp)
+#
+# Will compile the list of files together and link against clad.
+# Produces a binary named 'basename(test_dirname)'.
+function(add_clad_unittest test_dirname)
+  add_unittest(CladUnitTests ${test_dirname} ${ARGN})
+
+  # Remove the llvm_gtest_* coming from add_unittest.
+  get_target_property(GTEST_LINKED_LIBS ${test_dirname} LINK_LIBRARIES)
+  list(REMOVE_ITEM GTEST_LINKED_LIBS llvm_gtest_main llvm_gtest)
+  set_property(TARGET ${test_dirname} PROPERTY LINK_LIBRARIES ${GTEST_LINKED_LIBS})
+
+  enable_clad_for_executable(${test_dirname})
+  #target_include_directories(${test_dirname})
+  set(gtest_libs gtest gtest_main)
+  # Clang prior than clang13 (I think) merges both gmock into gtest.
+  if (TARGET gmock)
+    list(APPEND gtest_libs gmock gmock_main)
+  endif()
+
+  target_link_libraries(${test_dirname} PUBLIC ${gtest_libs})
+
+endfunction()
+
+add_subdirectory(Basic)
+