diff --git a/game.c b/game.c
index 9aa8b69..dbbb2cb 100644
--- a/game.c
+++ b/game.c
@@ -36,6 +36,7 @@
 #define MAX_SHAPES 256U
 #define MAX_ENTITIES 128U
 #define MAX_SHAPES_PER_ENTITY 2
+#define MAX_COLLISIONS 128U
 
 #define NELEMS(arr) (sizeof(arr) / sizeof(arr[0]))
 
@@ -65,6 +66,9 @@ typedef struct {
 	uint8_t flags;
 } shape_t;
 
+typedef struct {
+	unsigned entities[2];
+} collision_t;
 
 static const vec2_t ship_verts[] = {
 	{ 0.0, 0.11 },
@@ -236,6 +240,25 @@ static void ship_update()
 	fire->verts[0].y = FIRE_MEAN + FIRE_JITTER * rng_plusminus();
 }
 
+static unsigned check_collisions(collision_t *out)
+{
+	unsigned count = 0;
+	for (unsigned i = 0; i < entity_count; ++i) {
+		const entity_t *a = entities + i;
+		for (unsigned j = i + 1; j < entity_count; ++j) {
+			const entity_t *b = entities + j;
+			const float r = vec2_len(vec2_sub(b->pos, a->pos));
+			if (r <= a->collision_radius + b->collision_radius) {
+				assert(count < MAX_COLLISIONS);
+				collision_t *col = out + count++;
+				col->entities[0] = i;
+				col->entities[1] = j;
+			}
+		}
+	}
+	return count;
+}
+
 void game_init(float _aspect)
 {
 	input_on_shoot(shoot);
@@ -267,6 +290,7 @@ void game_init(float _aspect)
 
 void game_update()
 {
+
 	ship_update();
 
 	for (unsigned i = 0; i < entity_count; ++i) {
@@ -294,6 +318,9 @@ void game_update()
 		transforms[i]
 		    = mat3_mul_mat3(mat3_translation(e->pos), mat2_extend(e->dir));
 	}
+
+	static collision_t collisions[MAX_COLLISIONS];
+	const unsigned ncols = check_collisions(collisions);
 }
 
 void game_draw()