diff --git a/README.md b/README.md
index 8bc5dc7..09401a5 100644
--- a/README.md
+++ b/README.md
@@ -90,3 +90,9 @@ An obvious performance improvement would be to add some group bounding, so we do
 
 In any event, flamegraphs are a fun way to see what is going on.
 ![](images/glass_teapot_flamegraph.svg)
+
+## main/csg
+
+Add some constructive solid geometry - aka CSG...this is a sphere with a cubice bite taken out of it.
+
+![](images/csg.png)
\ No newline at end of file
diff --git a/images/csg.png b/images/csg.png
new file mode 100644
index 0000000..a08e8b8
Binary files /dev/null and b/images/csg.png differ
diff --git a/main/CMakeLists.txt b/main/CMakeLists.txt
index 334e856..0bd8a68 100644
--- a/main/CMakeLists.txt
+++ b/main/CMakeLists.txt
@@ -87,3 +87,11 @@ target_link_libraries(render_obj PRIVATE
         module_patterns
         module_raytracer
         module_shapes)
+
+add_executable(csg
+        csg.c)
+
+target_link_libraries(csg PRIVATE
+        module_patterns
+        module_raytracer
+        module_shapes)
diff --git a/main/csg.c b/main/csg.c
new file mode 100644
index 0000000..6f2f37e
--- /dev/null
+++ b/main/csg.c
@@ -0,0 +1,115 @@
+#include "logger.h"
+
+#include "canvas.h"
+#include "exceptions.h"
+#include "ray.h"
+#include <stdio.h>
+#include <sphere.h>
+#include <plane.h>
+#include <camera.h>
+#include "pattern.h"
+#include <math.h>
+#include <cube.h>
+#include <csg.h>
+
+CEXCEPTION_T e;
+void build_world(WORLD_World* world) {
+    TUPLES_Color red, green, blue;
+    TUPLES_init_color(&red, 1, 0, 0);
+    TUPLES_init_color(&green, 0, 1, 0);
+    TUPLES_init_color(&blue, 0, 0, 1);
+    PLANE_Plane* floor = PLANE_new();
+    WORLD_add_object(world, floor);
+    MATERIAL_Material* material = MATERIAL_new();
+    material->specular = 0;
+    PATTERN_Pattern* floor_pattern = PATTERN_new_stripe(&red, &green);
+    MATRIX_Matrix* pattern_transform = MATRIX_new_rotation_y(-M_PI / 5);
+    PATTERN_set_transform(floor_pattern, pattern_transform);
+    MATRIX_delete(pattern_transform);
+    MATERIAL_set_pattern(material, floor_pattern);
+    PATTERN_delete(floor_pattern);
+    PLANE_set_material(floor, material);
+    MATERIAL_delete(material);
+
+    SPHERE_Sphere* middle = SPHERE_new();
+    MATRIX_Matrix* middle_transform = MATRIX_new_translation(-0.5, 1, 0.5);
+    SPHERE_set_transform(middle, middle_transform);
+    MATRIX_delete(middle_transform);
+    MATERIAL_Material* middle_material = MATERIAL_new();
+    PATTERN_Pattern* middle_pattern = PATTERN_new_gradient(&red, &blue);
+    MATRIX_Matrix* middle_pattern_translate = MATRIX_new_translation(0.5, 0, 0);
+    MATRIX_Matrix* middle_pattern_scale = MATRIX_new_scaling(2, 2, 2);
+    MATRIX_Matrix* middle_pattern_transform = MATRIX_multiply(middle_pattern_scale, middle_pattern_translate);
+    PATTERN_set_transform(middle_pattern, middle_pattern_transform);
+    MATRIX_delete_all(middle_pattern_scale, middle_pattern_transform, middle_pattern_translate);
+    MATERIAL_set_pattern(middle_material, middle_pattern);
+    PATTERN_delete(middle_pattern);
+    middle_material->diffuse = 0.7;
+    middle_material->specular = 0.3;
+    SPHERE_set_material(middle, middle_material);
+    MATERIAL_delete(middle_material);
+
+    CUBE_Cube* cube = CUBE_new();
+    MATRIX_Matrix* cube_translate = MATRIX_new_translation(0.5, 0.5, 0.35);
+    MATRIX_Matrix* cube_rot = MATRIX_new_rotation_y(M_PI_4);
+    MATRIX_Matrix* cube_transform = MATRIX_multiply_many(cube_rot, cube_translate);
+    CUBE_set_transform(cube, cube_transform);
+    MATRIX_delete_all(cube_transform, cube_rot, cube_translate);
+    MATERIAL_Material* cube_mat = MATERIAL_new();
+    cube_mat->transparency = 1.0;
+    cube_mat->ambient = 0.0;
+    cube_mat->diffuse = 0.0;
+    cube_mat->specular = 0.0;
+    cube_mat->shadow_calc = false;
+    CUBE_set_material(cube, cube_mat);
+    MATERIAL_delete(cube_mat);
+
+    CSG_Csg* csg = CSG_new(CSG_Difference, middle, cube);
+    WORLD_add_object(world, csg);
+}
+
+int main(void) {
+    Try {
+                LOGGER_log(LOGGER_INFO, "Building world...\n");
+                TUPLES_Point* light_position = TUPLES_new_point(10, 10, -10);
+                TUPLES_Color* light_color = TUPLES_new_color(1, 1, 1);
+                LIGHTS_PointLight* light = LIGHTS_new_pointlight(light_position, light_color);
+                TUPLES_delete_all(light_position, light_color);
+
+                CAMERA_Camera* camera = CAMERA_new(1000, 500, M_PI / 3.0);
+                TUPLES_Point* from = TUPLES_new_point(0, 1.5, -5);
+                TUPLES_Point* to = TUPLES_new_point(0, 1, 0);
+                TUPLES_Vector* up = TUPLES_new_vector(0, 1, 0);
+                MATRIX_Matrix* camera_transform = CAMERA_view_transform(from, to, up);
+                CAMERA_set_transform(camera, camera_transform);
+                TUPLES_delete_all(from, to, up);
+                MATRIX_delete(camera_transform);
+
+                WORLD_World* world = WORLD_new(light);
+                build_world(world);
+                LOGGER_log(LOGGER_INFO, "Rendering...\n");
+                UTILITIES_Timer* render_timer = UTILITIES_Timer_start();
+                CANVAS_Canvas* canvas = CAMERA_render(camera, world);
+                UTILITIES_Timer_Results render_results = UTILITIES_Timer_stop(render_timer);
+
+                char *filename = "csg.ppm";
+                LOGGER_log(LOGGER_INFO, "Writing file %s...\n", filename);
+                CANVAS_write_to_file(canvas, filename);
+                LOGGER_log(LOGGER_INFO, "Cleaning up...\n");
+                WORLD_delete_all_objects(world);
+                WORLD_delete(world);
+                LIGHTS_delete_pointlight(light);
+                CAMERA_delete(camera);
+                CANVAS_delete(canvas);
+                LOGGER_log(LOGGER_INFO, "Wall: %.2f User: %.2f System: %.2f\n", render_results.wall_time_seconds,
+                           render_results.user_time_seconds, render_results.system_time_seconds);
+            } Catch(e) {
+        if (e == E_MALLOC_FAILED)
+            printf("Malloc failed.  Exiting\n");
+        else if (e == E_FILE_FAILED)
+            printf("Failed to open test.ppm\n");
+        else
+            printf("Unknown exception %i occurred\n", e);
+    }
+    return 0;
+}
diff --git a/module_datastructures/arrlist.c b/module_datastructures/arrlist.c
index 2a79634..7d1e713 100644
--- a/module_datastructures/arrlist.c
+++ b/module_datastructures/arrlist.c
@@ -33,7 +33,12 @@ void ARRLIST_add(ARRLIST_List* list, void* object) {
     list->count++;
 }
 
-void* ARRLIST_safe_get(ARRLIST_List* list, unsigned int item) {
+unsigned int ARRLIST_item_count(const ARRLIST_List* list) {
+    assert(list);
+    return list->count;
+}
+
+void* ARRLIST_safe_get(const ARRLIST_List* list, unsigned int item) {
     assert(list);
     if (item >= list->count) {
         Throw(E_INDEX_OUT_OF_BOUNDS);
diff --git a/module_datastructures/arrlist.h b/module_datastructures/arrlist.h
index 8386f12..a3be274 100644
--- a/module_datastructures/arrlist.h
+++ b/module_datastructures/arrlist.h
@@ -15,7 +15,9 @@ void ARRLIST_add(ARRLIST_List* list, void* object);
  * @param item
  * @return
  */
-void* ARRLIST_safe_get(ARRLIST_List* list, unsigned int item);
+void* ARRLIST_safe_get(const ARRLIST_List* list, unsigned int item);
+
+unsigned int ARRLIST_item_count(const ARRLIST_List* list);
 
 /**
  * Removes an item from the list and moves the remaining ptrs up.
diff --git a/module_raytracer/arrlist.c b/module_raytracer/arrlist.c
deleted file mode 100644
index 2a79634..0000000
--- a/module_raytracer/arrlist.c
+++ /dev/null
@@ -1,98 +0,0 @@
-#include <stdlib.h>
-#include <assert.h>
-
-#include "exceptions.h"
-
-#include "arrlist.h"
-
-typedef struct ARRLIST_List {
-    void** items;
-    unsigned int count;
-} ARRLIST_List;
-
-ARRLIST_List* ARRLIST_new() {
-    ARRLIST_List* list = malloc(sizeof(ARRLIST_List));
-    if (!list) {
-        Throw(E_MALLOC_FAILED);
-    }
-    list->items = NULL;
-    list->count = 0;
-    return list;
-}
-
-void ARRLIST_add(ARRLIST_List* list, void* object) {
-    assert(list);
-    assert(object);
-    void** tmpptr = reallocarray(list->items, sizeof(void*), list->count + 1);
-    if (!tmpptr) {
-        Throw(E_MALLOC_FAILED);
-    } else {
-        list->items = tmpptr;
-    }
-    list->items[list->count] = object;
-    list->count++;
-}
-
-void* ARRLIST_safe_get(ARRLIST_List* list, unsigned int item) {
-    assert(list);
-    if (item >= list->count) {
-        Throw(E_INDEX_OUT_OF_BOUNDS);
-    }
-    return list->items[item];
-}
-
-void ARRLIST_remove(ARRLIST_List* list, const void* object) {
-    assert(list);
-    assert(object);
-    bool found = false;
-    for (unsigned int ndx = 0; ndx < list->count; ndx++) {
-        if (list->items[ndx] == object) {
-            found = true;
-        }
-        if (found) {
-            if (ndx + 1 != list->count) {
-                list->items[ndx] = list->items[ndx + 1];
-            } else {
-                list->items[ndx] = NULL;
-            }
-        }
-    }
-    if (found) {
-        list->count--;
-    }
-}
-
-void ARRLIST_iterator(ARRLIST_List* list, void (*apply_each_ptr)(void* ptr, void* context), void* context) {
-    assert(list);
-    assert(apply_each_ptr);
-    for (unsigned int ndx = 0; ndx < list->count; ndx++) {
-        apply_each_ptr(list->items[ndx], context);
-    }
-}
-
-bool ARRLIST_is_empty(const ARRLIST_List* list) {
-    assert(list);
-    return (list->count == 0);
-}
-
-bool ARRLIST_contains(const ARRLIST_List* list, const void* object) {
-    assert(list);
-    assert(object);
-    for (unsigned int ndx = 0; ndx < list->count; ndx++) {
-        if (list->items[ndx] == object) {
-            return true;
-        }
-    }
-    return false;
-}
-
-void* ARRLIST_last(const ARRLIST_List* list) {
-    assert(list);
-    return list->items[list->count - 1];
-}
-
-void ARRLIST_delete(ARRLIST_List* list) {
-    assert(list);
-    free(list->items);
-    free(list);
-}
diff --git a/module_raytracer/arrlist.h b/module_raytracer/arrlist.h
deleted file mode 100644
index 8386f12..0000000
--- a/module_raytracer/arrlist.h
+++ /dev/null
@@ -1,54 +0,0 @@
-#ifndef DATA_STRUCTURES_ARRLIST_H
-#define DATA_STRUCTURES_ARRLIST_H
-
-#include <stdbool.h>
-
-typedef struct ARRLIST_List ARRLIST_List;
-
-ARRLIST_List* ARRLIST_new();
-void ARRLIST_add(ARRLIST_List* list, void* object);
-
-/**
- * gets a point to an item in the list.  throws a E_INDEX_OUT_OF_BOUNDS
- * when an item that doesn't exist is accessed.
- * @param list
- * @param item
- * @return
- */
-void* ARRLIST_safe_get(ARRLIST_List* list, unsigned int item);
-
-/**
- * Removes an item from the list and moves the remaining ptrs up.
- * Doesn't bother to resize the array.
- * @param list
- * @param object
- */
-void ARRLIST_remove(ARRLIST_List* list, const void* object);
-
-bool ARRLIST_is_empty(const ARRLIST_List* list);
-
-void ARRLIST_iterator(ARRLIST_List* list, void (*apply_each_ptr)(void* ptr, void* context), void* context);
-
-/**
- * Performs linear search of the list.  Comparing only the ptr values!
- * @param list
- * @param object
- * @return
- */
-bool ARRLIST_contains(const ARRLIST_List* list, const void* object);
-
-/**
- * Returns the last item in the list
- * @param ARRLIST_List
- * @return
- */
-void* ARRLIST_last(const ARRLIST_List* list);
-
-/**
- * Frees the list and backing array, but doesn't free anything pointed to
- * the ptrs that have been added to the list.
- * @param list
- */
-void ARRLIST_delete(ARRLIST_List* list);
-
-#endif //DATA_STRUCTURES_ARRLIST_H
diff --git a/module_shapes/CMakeLists.txt b/module_shapes/CMakeLists.txt
index 5196abd..b1358d2 100644
--- a/module_shapes/CMakeLists.txt
+++ b/module_shapes/CMakeLists.txt
@@ -7,18 +7,21 @@ add_library(module_shapes STATIC
         cylinder.c cylinder.h
         cone.c cone.h
         group.c group.h
-        triangle.c triangle.h)
+        triangle.c triangle.h
+        csg.c csg.h)
 
 target_include_directories(module_shapes PUBLIC
         ${CMAKE_CURRENT_LIST_DIR}
         module_raytracer
         module_utilities
+        module_datastructures
         CException
 )
 
 target_link_libraries(module_shapes
         module_raytracer
         module_utilities
+        module_datastructures
         CException
         m
 )
diff --git a/module_shapes/cone.c b/module_shapes/cone.c
index 135697c..1d39897 100644
--- a/module_shapes/cone.c
+++ b/module_shapes/cone.c
@@ -7,7 +7,8 @@
 const SHAPE_vtable CONE_vtable = {
         &CONE_local_intersect,
         &CYLINDER_delete_shape,
-        &CONE_local_normal_at
+        &CONE_local_normal_at,
+        &SHAPE_default_shape_contains
 };
 
 CONE_Cone* CONE_new() {
diff --git a/module_shapes/csg.c b/module_shapes/csg.c
new file mode 100644
index 0000000..a9a2e89
--- /dev/null
+++ b/module_shapes/csg.c
@@ -0,0 +1,124 @@
+#include <assert.h>
+#include "csg.h"
+
+bool CSG_shape_contains(const SHAPE_Shape* a, const SHAPE_Shape* b) {
+    assert(a);
+    assert(b);
+    CSG_Csg* acsg = (CSG_Csg*) a;
+    return acsg->left->vtable->contains(acsg->left, b) ||
+           acsg->right->vtable->contains(acsg->right, b);
+}
+
+const SHAPE_vtable CSG_vtable = {
+        &CSG_local_intersect,
+        &CSG_delete_shape,
+        &CSG_local_normal_at,
+        &CSG_shape_contains
+};
+
+bool CSG_intersection_allowed(CSG_Operation op, bool lhit, bool inl, bool inr) {
+    switch(op) {
+        case CSG_Union:
+            return (lhit && !inr) || (!lhit && !inl);
+        case CSG_Intersection:
+            return (lhit && inr) || (!lhit && inl);
+        case CSG_Difference:
+            return (lhit && !inr) || (!lhit && inl);
+    }
+    assert(0);  // should never get here
+    return false;
+}
+
+RAY_Intersections* CSG_filter_intersections(const CSG_Csg* csg, const RAY_Intersections* intersections) {
+    assert(csg);
+    assert(intersections);
+
+    bool inl = false;
+    bool inr = false;
+
+    RAY_Intersections* result = RAY_new_intersections();
+
+    for (uint i=0; i<intersections->count; i++) {
+        const RAY_Xs* xs = &intersections->xs[i];
+        bool lhit = csg->left->vtable->contains(csg->left, xs->object);
+
+        if (CSG_intersection_allowed(csg->operation, lhit, inl, inr)) {
+            RAY_add_intersection_tri(result, xs->t, xs->object, xs->u, xs->v);
+        }
+
+        if (lhit) {
+            inl = !inl;
+        } else {
+            inr = !inr;
+        }
+    }
+
+    return result;
+}
+
+CSG_Csg* CSG_new(CSG_Operation op, SHAPE_Shape* left, SHAPE_Shape* right) {
+    assert(left);
+    assert(right);
+    CSG_Csg* csg = malloc(sizeof(CSG_Csg));
+    if (!csg) {
+        Throw(E_MALLOC_FAILED);
+    }
+    CSG_init(csg, op, left, right);
+    return csg;
+}
+
+void CSG_init(CSG_Csg* csg, CSG_Operation op, SHAPE_Shape* left, SHAPE_Shape* right) {
+    assert(csg);
+    assert(left);
+    assert(right);
+    SHAPE_init(&csg->shape, &CSG_vtable);
+    csg->left = left;
+    SHAPE_set_parent(left, csg);
+    csg->right = right;
+    SHAPE_set_parent(right, csg);
+    csg->operation = op;
+}
+
+void CSG_destroy(CSG_Csg* csg) {
+    assert(csg);
+    SHAPE_destroy(&csg->shape);
+}
+
+void CSG_delete(CSG_Csg* csg) {
+    assert(csg);
+    CSG_destroy(csg);
+    free(csg);
+}
+
+void CSG_delete_shape(SHAPE_Shape* shape) {
+    assert(shape);
+    CSG_Csg* csg = (CSG_Csg*) shape;
+    CSG_delete(csg);
+}
+
+void CSG_local_normal_at(TUPLES_Vector* local_normal, SHAPE_Shape* csg, const TUPLES_Point* local_point, const RAY_Xs* hit) {
+    assert(local_normal);
+    assert(csg);
+    assert(local_point);
+    assert(hit);
+    UNUSED(local_normal);
+    UNUSED(csg);
+    UNUSED(local_point);
+    UNUSED(hit);
+    assert(0);
+}
+
+void CSG_local_intersect(RAY_Intersections* intersections, SHAPE_Shape* shape, const RAY_Ray* local_ray) {
+    assert(intersections);
+    assert(shape);
+    assert(local_ray);
+    CSG_Csg* csg = (CSG_Csg*) shape;
+    RAY_Intersections* temp_xs = RAY_new_intersections();
+    SHAPE_intersect(temp_xs, csg->left, local_ray);
+    SHAPE_intersect(temp_xs, csg->right, local_ray);
+    RAY_sort_intersections(temp_xs);
+    RAY_Intersections* filtered_xs = CSG_filter_intersections(csg, temp_xs);
+    RAY_add_intersections(intersections, filtered_xs);
+    RAY_delete_intersections(temp_xs);
+    RAY_delete_intersections(filtered_xs);
+}
diff --git a/module_shapes/csg.h b/module_shapes/csg.h
new file mode 100644
index 0000000..475586e
--- /dev/null
+++ b/module_shapes/csg.h
@@ -0,0 +1,35 @@
+#ifndef SIMPLE_RAYTRACER_CSG_H
+#define SIMPLE_RAYTRACER_CSG_H
+
+#include "shape.h"
+
+typedef enum CSG_Operation {
+    CSG_Union,
+    CSG_Intersection,
+    CSG_Difference
+} CSG_Operation;
+
+/**
+ * \extends SHAPE_Shape
+ */
+typedef struct CSG_Csg {
+    SHAPE_Shape         shape;
+    SHAPE_Shape*        left;
+    SHAPE_Shape*        right;
+    CSG_Operation       operation;
+} CSG_Csg;
+
+const SHAPE_vtable CSG_vtable;
+
+CSG_Csg* CSG_new(CSG_Operation op, SHAPE_Shape* left, SHAPE_Shape* right);
+void CSG_init(CSG_Csg* csg, CSG_Operation op, SHAPE_Shape* left, SHAPE_Shape* right);
+void CSG_destroy(CSG_Csg* csg);
+void CSG_delete(CSG_Csg* csg);
+void CSG_delete_shape(SHAPE_Shape* shape);
+
+void CSG_local_normal_at(TUPLES_Vector* local_normal, SHAPE_Shape* csg, const TUPLES_Point* local_point, const RAY_Xs* hit);
+void CSG_local_intersect(RAY_Intersections* intersections, SHAPE_Shape* csg, const RAY_Ray* local_ray);
+bool CSG_intersection_allowed(CSG_Operation op, bool lhit, bool inl, bool inr);
+RAY_Intersections* CSG_filter_intersections(const CSG_Csg* csg, const RAY_Intersections* intersections);
+
+#endif //SIMPLE_RAYTRACER_CSG_H
diff --git a/module_shapes/cube.c b/module_shapes/cube.c
index adf72fd..40e4666 100644
--- a/module_shapes/cube.c
+++ b/module_shapes/cube.c
@@ -6,7 +6,8 @@
 const SHAPE_vtable CUBE_vtable = {
         &CUBE_local_intersect,
         &SHAPE_delete,
-        &CUBE_local_normal_at
+        &CUBE_local_normal_at,
+        &SHAPE_default_shape_contains
 };
 
 static double max(double x, double y, double z) {
diff --git a/module_shapes/cylinder.c b/module_shapes/cylinder.c
index 9af91fd..ecfe134 100644
--- a/module_shapes/cylinder.c
+++ b/module_shapes/cylinder.c
@@ -13,7 +13,8 @@ void CYLINDER_delete_shape(SHAPE_Shape* shape) {
 const SHAPE_vtable CYLINDER_vtable = {
         &CYLINDER_local_intersect,
         &CYLINDER_delete_shape,
-        &CYLINDER_local_normal_at
+        &CYLINDER_local_normal_at,
+        &SHAPE_default_shape_contains
 };
 
 CYLINDER_Cylinder* CYLINDER_new() {
diff --git a/module_shapes/group.c b/module_shapes/group.c
index e886519..c877609 100644
--- a/module_shapes/group.c
+++ b/module_shapes/group.c
@@ -1,11 +1,25 @@
 #include <assert.h>
 #include <exceptions.h>
+#include <arrlist.h>
 #include "group.h"
 
+bool GROUP_shape_contains(const SHAPE_Shape* a, const SHAPE_Shape* b) {
+    if (a == b) return true;
+    GROUP_Group* a_group = (GROUP_Group*) a;
+    for (unsigned int i=0; i < ARRLIST_item_count(a_group->list); i++) {
+        SHAPE_Shape* shape = (SHAPE_Shape*)ARRLIST_safe_get(a_group->list, i);
+        if (shape->vtable->contains(shape, b)) {
+            return true;
+        }
+    }
+    return false;
+}
+
 const SHAPE_vtable GROUP_vtable = {
         &GROUP_local_intersect,
         &GROUP_delete_shape,
-        &GROUP_local_normal_at
+        &GROUP_local_normal_at,
+        &GROUP_shape_contains
 };
 
 GROUP_Group* GROUP_new() {
diff --git a/module_shapes/plane.c b/module_shapes/plane.c
index c2109f9..b72b6d1 100644
--- a/module_shapes/plane.c
+++ b/module_shapes/plane.c
@@ -6,7 +6,8 @@
 const SHAPE_vtable PLANE_vtable = {
         &PLANE_local_intersect,
         &SHAPE_delete,
-        &PLANE_local_normal_at
+        &PLANE_local_normal_at,
+        &SHAPE_default_shape_contains
 };
 
 void PLANE_local_normal_at(TUPLES_Vector* local_normal, SHAPE_Shape* shape, const TUPLES_Point* local_point, const RAY_Xs* hit) {
diff --git a/module_shapes/shape.c b/module_shapes/shape.c
index 67cf13a..b89a74c 100644
--- a/module_shapes/shape.c
+++ b/module_shapes/shape.c
@@ -147,3 +147,9 @@ void SHAPE_normal_to_world(TUPLES_Vector* result, const SHAPE_Shape* shape, cons
         SHAPE_normal_to_world(result, parent, result);
     }
 }
+
+bool SHAPE_default_shape_contains(const SHAPE_Shape* a, const SHAPE_Shape* b) {
+    assert(a);
+    assert(b);
+    return a == b;
+}
diff --git a/module_shapes/shape.h b/module_shapes/shape.h
index 9ac9830..5d287e5 100644
--- a/module_shapes/shape.h
+++ b/module_shapes/shape.h
@@ -14,6 +14,7 @@ typedef struct SHAPE_vtable {
     void (*local_intersect)(RAY_Intersections* intersections, SHAPE_Shape* shape, const RAY_Ray* ray);
     void (*delete)(SHAPE_Shape*);
     void (*local_normal_at)(TUPLES_Vector* local_normal, SHAPE_Shape* shape, const TUPLES_Point* local_point, const RAY_Xs* hit);
+    bool (*contains)(const SHAPE_Shape* a, const SHAPE_Shape* b);
 } SHAPE_vtable;
 
 /**
@@ -65,4 +66,6 @@ void SHAPE_normal_at(TUPLES_Vector* world_normal, SHAPE_Shape* shape, const TUPL
 void SHAPE_world_to_object(TUPLES_Point* result, const SHAPE_Shape* shape, const TUPLES_Point* world_point);
 void SHAPE_normal_to_world(TUPLES_Vector* result, const SHAPE_Shape* shape, const TUPLES_Vector* normal);
 
+bool SHAPE_default_shape_contains(const SHAPE_Shape* a, const SHAPE_Shape* b);
+
 #endif //DATA_STRUCTURES_SHAPE_H
diff --git a/module_shapes/sphere.c b/module_shapes/sphere.c
index 2b70774..e55e7d8 100644
--- a/module_shapes/sphere.c
+++ b/module_shapes/sphere.c
@@ -5,7 +5,8 @@
 const SHAPE_vtable SPHERE_vtable = {
         &SPHERE_local_intersect,
         &SHAPE_delete,
-        &SPHERE_local_normal_at
+        &SPHERE_local_normal_at,
+        &SHAPE_default_shape_contains
 };
 
 void SPHERE_local_intersect(RAY_Intersections* intersections, SHAPE_Shape* shape, const RAY_Ray* local_ray) {
diff --git a/module_shapes/testshape.c b/module_shapes/testshape.c
index 1493480..119004b 100644
--- a/module_shapes/testshape.c
+++ b/module_shapes/testshape.c
@@ -5,7 +5,8 @@
 const SHAPE_vtable TESTSHAPE_vtable = {
         &TESTSHAPE_local_intersect,
         &SHAPE_delete,
-        &TESTSHAPE_local_normal_at
+        &TESTSHAPE_local_normal_at,
+        &SHAPE_default_shape_contains
 };
 
 void TESTSHAPE_init(TESTSHAPE_TestShape* shape) {
diff --git a/module_shapes/triangle.c b/module_shapes/triangle.c
index a095422..905706c 100644
--- a/module_shapes/triangle.c
+++ b/module_shapes/triangle.c
@@ -5,13 +5,15 @@
 const SHAPE_vtable TRIANGLE_vtable = {
         &TRIANGLE_local_intersect,
         &TRIANGLE_delete_shape,
-        &TRIANGLE_local_normal_at
+        &TRIANGLE_local_normal_at,
+        &SHAPE_default_shape_contains
 };
 
 const SHAPE_vtable TRIANGLE_smooth_vtable = {
        &TRIANGLE_local_intersect,
        &TRIANGLE_delete_shape,
-       &TRIANGLE_smooth_local_normal_at
+       &TRIANGLE_smooth_local_normal_at,
+       &SHAPE_default_shape_contains
 };
 
 TRIANGLE_Triangle* TRIANGLE_new_from_points(const TUPLES_Point* p1, const TUPLES_Point* p2, const TUPLES_Point* p3) {
diff --git a/test/module_datastructures/test_arrlist.c b/test/module_datastructures/test_arrlist.c
index e8e1bbb..6fbb075 100644
--- a/test/module_datastructures/test_arrlist.c
+++ b/test/module_datastructures/test_arrlist.c
@@ -104,5 +104,6 @@ int main(void) {
     RUN_TEST(test_arrlist_remove);
     RUN_TEST(test_arrlist_last);
     RUN_TEST(test_arrlist_safe_get_should_throw_on_bad_index);
+    RUN_TEST(test_arrlist_safe_get);
     UNITY_END();
 }
diff --git a/test/module_shapes/CMakeLists.txt b/test/module_shapes/CMakeLists.txt
index db46d00..bc59c5b 100644
--- a/test/module_shapes/CMakeLists.txt
+++ b/test/module_shapes/CMakeLists.txt
@@ -63,3 +63,11 @@ target_link_libraries(module_shapes_triangle
         Unity
         m)
 add_test(shapes_triangle module_shapes_triangle)
+
+add_executable(module_shapes_csg
+        test_csg.c)
+target_link_libraries(module_shapes_csg
+        module_shapes
+        Unity
+        m)
+add_test(shapes_csg module_shapes_csg)
diff --git a/test/module_shapes/test_csg.c b/test/module_shapes/test_csg.c
new file mode 100644
index 0000000..0027769
--- /dev/null
+++ b/test/module_shapes/test_csg.c
@@ -0,0 +1,142 @@
+#include <unity.h>
+#include <csg.h>
+#include <sphere.h>
+#include <cube.h>
+
+void setUp() {}
+void tearDown() {}
+
+void test_create_csg() {
+    SPHERE_Sphere* s = SPHERE_new();
+    CUBE_Cube* c = CUBE_new();
+    CSG_Csg* csg = CSG_new(CSG_Union, s, c);
+
+    TEST_ASSERT_EQUAL(CSG_Union, csg->operation);
+    TEST_ASSERT_EQUAL_PTR(c, csg->right);
+    TEST_ASSERT_EQUAL_PTR(s, csg->left);
+    TEST_ASSERT_EQUAL_PTR(csg, SHAPE_get_parent(s));
+    TEST_ASSERT_EQUAL_PTR(csg, SHAPE_get_parent(c));
+
+    SPHERE_delete(s);
+    CUBE_delete(c);
+    CSG_delete(csg);
+}
+
+void test_csg_intersection_allowed(CSG_Operation op, bool lhit, bool inl, bool inr, bool expected) {
+    bool result = CSG_intersection_allowed(op, lhit, inl, inr);
+    if (expected != result) {
+        printf("Testing intersection allowed op(%u) lhit(%d) inl(%d) inr(%d) expected(%d) got(%d)\n", op, lhit, inl, inr, expected, result);
+        TEST_FAIL();
+    }
+}
+
+void test_csg_intersection_allowed_union_op() {
+    test_csg_intersection_allowed(CSG_Union, true, true, true, false);
+    test_csg_intersection_allowed(CSG_Union, true, true, false, true);
+    test_csg_intersection_allowed(CSG_Union, true, false, true, false);
+    test_csg_intersection_allowed(CSG_Union, true, false, false, true);
+    test_csg_intersection_allowed(CSG_Union, false, true, true, false);
+    test_csg_intersection_allowed(CSG_Union, false, true, false, false);
+    test_csg_intersection_allowed(CSG_Union, false, false, true, true);
+    test_csg_intersection_allowed(CSG_Union, false, false, false, true);
+
+    test_csg_intersection_allowed(CSG_Intersection, true, true, true, true);
+    test_csg_intersection_allowed(CSG_Intersection, true, true, false, false);
+    test_csg_intersection_allowed(CSG_Intersection, true, false, true, true);
+    test_csg_intersection_allowed(CSG_Intersection, true, false, false, false);
+    test_csg_intersection_allowed(CSG_Intersection, false, true, true, true);
+    test_csg_intersection_allowed(CSG_Intersection, false, true, false, true);
+    test_csg_intersection_allowed(CSG_Intersection, false, false, true, false);
+    test_csg_intersection_allowed(CSG_Intersection, false, false, false, false);
+
+    test_csg_intersection_allowed(CSG_Difference, true, true, true, false);
+    test_csg_intersection_allowed(CSG_Difference, true, true, false, true);
+    test_csg_intersection_allowed(CSG_Difference, true, false, true, false);
+    test_csg_intersection_allowed(CSG_Difference, true, false, false, true);
+    test_csg_intersection_allowed(CSG_Difference, false, true, true, true);
+    test_csg_intersection_allowed(CSG_Difference, false, true, false, true);
+    test_csg_intersection_allowed(CSG_Difference, false, false, true, false);
+    test_csg_intersection_allowed(CSG_Difference, false, false, false, false);
+}
+
+void filter_list_of_intersections(CSG_Operation op, uint first, uint second) {
+    SPHERE_Sphere* s1 = SPHERE_new();
+    CUBE_Cube* s2 = CUBE_new();
+    CSG_Csg* csg = CSG_new(op, s1, s2);
+    RAY_Intersections* xs = RAY_new_intersections();
+    RAY_add_intersection(xs, 1, s1);
+    RAY_add_intersection(xs, 2, s2);
+    RAY_add_intersection(xs, 3, s1);
+    RAY_add_intersection(xs, 4, s2);
+    RAY_Intersections* filtered_xs = CSG_filter_intersections(csg, xs);
+    TEST_ASSERT_EQUAL(2, filtered_xs->count);
+    TEST_ASSERT_EQUAL_PTR(xs->xs[first].object, filtered_xs->xs[0].object);
+    TEST_ASSERT_EQUAL_DOUBLE(xs->xs[first].t, filtered_xs->xs[0].t);
+    TEST_ASSERT_EQUAL_PTR(xs->xs[second].object, filtered_xs->xs[1].object);
+    TEST_ASSERT_EQUAL_DOUBLE(xs->xs[second].t, filtered_xs->xs[1].t);
+    RAY_delete_intersections(xs);
+    RAY_delete_intersections(filtered_xs);
+    CSG_delete(csg);
+    SPHERE_delete(s1);
+    CUBE_delete(s2);
+}
+
+void test_filter_list_of_intersections() {
+    filter_list_of_intersections(CSG_Union, 0, 3);
+    filter_list_of_intersections(CSG_Intersection, 1, 2);
+    filter_list_of_intersections(CSG_Difference, 0, 1);
+}
+
+void test_ray_misses_a_csg_object() {
+    SPHERE_Sphere* s = SPHERE_new();
+    CUBE_Cube* c = CUBE_new();
+    CSG_Csg* csg = CSG_new(CSG_Union, s, c);
+
+    RAY_Ray ray;
+    RAY_init(&ray, 0, 2, -5, 0, 0, 1);
+    RAY_Intersections *xs = RAY_new_intersections();
+
+    CSG_local_intersect(xs, (SHAPE_Shape*)csg, &ray);
+
+    TEST_ASSERT_EQUAL(0, xs->count);
+    RAY_delete_intersections(xs);
+    CSG_delete(csg);
+    CUBE_delete(c);
+    SPHERE_delete(s);
+}
+
+void test_ray_hits_csg_object() {
+    SPHERE_Sphere* s1 = SPHERE_new();
+    SPHERE_Sphere* s2 = SPHERE_new();
+    MATRIX_Matrix* s2_transform = MATRIX_new_translation(0, 0, 0.5);
+    SPHERE_set_transform(s2, s2_transform);
+    CSG_Csg* csg = CSG_new(CSG_Union, s1, s2);
+    MATRIX_delete(s2_transform);
+
+    RAY_Ray ray;
+    RAY_init(&ray, 0, 0, -5, 0, 0, 1);
+    RAY_Intersections* xs = RAY_new_intersections();
+
+    CSG_local_intersect(xs, (SHAPE_Shape*)csg, &ray);
+
+    TEST_ASSERT_EQUAL(2, xs->count);
+    TEST_ASSERT_EQUAL_DOUBLE(4, xs->xs[0].t);
+    TEST_ASSERT_EQUAL_PTR(s1, xs->xs[0].object);
+    TEST_ASSERT_EQUAL_DOUBLE(6.5, xs->xs[1].t);
+    TEST_ASSERT_EQUAL_PTR(s2, xs->xs[1].object);
+
+    RAY_delete_intersections(xs);
+    CSG_delete(csg);
+    SPHERE_delete_all(s1, s2);
+}
+
+int main(void)
+{
+    UNITY_BEGIN();
+    RUN_TEST(test_create_csg);
+    RUN_TEST(test_csg_intersection_allowed_union_op);
+    RUN_TEST(test_filter_list_of_intersections);
+    RUN_TEST(test_ray_misses_a_csg_object);
+    RUN_TEST(test_ray_hits_csg_object);
+    return UNITY_END();
+}