diff --git a/src/assembly.rs b/src/assembly.rs
new file mode 100644
index 0000000..60b4b94
--- /dev/null
+++ b/src/assembly.rs
@@ -0,0 +1,45 @@
+use std::collections::HashMap;
+
+use math::Matrix4x4;
+use bvh::BVH;
+use surface::Surface;
+
+
+#[derive(Debug)]
+pub struct Assembly {
+    // Instance list
+    instances: Vec<Instance>,
+    xforms: Vec<Matrix4x4>,
+
+    // Object list
+    objects: Vec<Object>,
+    object_map: HashMap<String, usize>, // map Name -> Index
+
+    // Assembly list
+    assemblies: Vec<Assembly>,
+    assembly_map: HashMap<String, usize>, // map Name -> Index
+
+    // Object accel
+    object_accel: BVH,
+}
+
+
+#[derive(Debug)]
+pub enum Object {
+    Surface(Box<Surface>),
+}
+
+
+#[derive(Debug, Copy, Clone)]
+enum Instance {
+    Object {
+        data_index: usize,
+        transform_indices: (usize, usize),
+        shader_index: usize,
+    },
+
+    Assembly {
+        data_index: usize,
+        transform_indices: (usize, usize),
+    },
+}
diff --git a/src/main.rs b/src/main.rs
index 075f708..31da264 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -14,6 +14,8 @@ mod image;
 mod triangle;
 mod surface;
 mod bvh;
+mod scene;
+mod assembly;
 mod halton;
 
 use std::mem;
diff --git a/src/scene.rs b/src/scene.rs
new file mode 100644
index 0000000..1e748f7
--- /dev/null
+++ b/src/scene.rs
@@ -0,0 +1,11 @@
+use camera::Camera;
+use assembly::Assembly;
+
+
+#[derive(Debug)]
+pub struct Scene {
+    name: String,
+    background_color: (f32, f32, f32),
+    camera: Camera,
+    root: Assembly,
+}
diff --git a/src/surface/mod.rs b/src/surface/mod.rs
index 78c04e4..f38e5e7 100644
--- a/src/surface/mod.rs
+++ b/src/surface/mod.rs
@@ -1,11 +1,14 @@
 #![allow(dead_code)]
 
+use std::fmt::Debug;
+
 pub mod triangle_mesh;
 
 use ray::Ray;
 use math::{Point, Normal, Matrix4x4};
 
 
+#[derive(Debug)]
 pub enum SurfaceIntersection {
     Miss,
     Occlude,
@@ -18,6 +21,6 @@ pub enum SurfaceIntersection {
     },
 }
 
-pub trait Surface {
+pub trait Surface: Debug {
     fn intersect_rays(&self, rays: &mut [Ray], isects: &mut [SurfaceIntersection]);
 }
diff --git a/src/surface/triangle_mesh.rs b/src/surface/triangle_mesh.rs
index 80269dc..7e8c6f2 100644
--- a/src/surface/triangle_mesh.rs
+++ b/src/surface/triangle_mesh.rs
@@ -9,6 +9,7 @@ use bvh::BVH;
 
 use super::{Surface, SurfaceIntersection};
 
+#[derive(Debug)]
 pub struct TriangleMesh {
     time_samples: usize,
     geo: Vec<(Point, Point, Point)>,