diff --git a/mem_arena/src/lib.rs b/mem_arena/src/lib.rs
index 33a125b..65a00a9 100644
--- a/mem_arena/src/lib.rs
+++ b/mem_arena/src/lib.rs
@@ -71,6 +71,8 @@ impl MemArena {
     ///
     /// CAUTION: the memory returned is uninitialized.  Make sure to initalize before using!
     pub unsafe fn alloc_uninitialized<'a, T: Copy>(&'a self) -> &'a mut T {
+        assert!(size_of::<T>() > 0);
+
         let memory = self.alloc_raw(size_of::<T>(), align_of::<T>()) as *mut T;
 
         memory.as_mut().unwrap()
@@ -103,6 +105,8 @@ impl MemArena {
     /// Allocates memory for `len` values of type `T`, returning a mutable slice to it.
     /// All elements are initialized to the given `value`.
     pub unsafe fn alloc_array_uninitialized<'a, T: Copy>(&'a self, len: usize) -> &'a mut [T] {
+        assert!(size_of::<T>() > 0);
+
         let array_mem_size = {
             let alignment_padding = alignment_offset(size_of::<T>(), align_of::<T>());
             let aligned_type_size = size_of::<T>() + alignment_padding;
@@ -119,13 +123,17 @@ impl MemArena {
     /// CAUTION: this returns uninitialized memory.  Make sure to initialize the
     /// memory after calling.
     unsafe fn alloc_raw(&self, size: usize, alignment: usize) -> *mut u8 {
-        assert!(size > 0);
         assert!(alignment > 0);
 
         let mut blocks = self.blocks.borrow_mut();
 
+
+        // If it's a zero-size allocation, just point to the beginning of the curent block.
+        if size == 0 {
+            return blocks.first_mut().unwrap().as_mut_ptr();
+        }
         // If the desired size is considered a "large allocation", give it its own memory block.
-        if size > self.large_alloc_threshold {
+        else if size > self.large_alloc_threshold {
             blocks.push(Vec::with_capacity(size + alignment - 1));
             blocks.last_mut().unwrap().set_len(size + alignment - 1);
 
diff --git a/src/parse/psy.rs b/src/parse/psy.rs
index f9aeb78..9fdc817 100644
--- a/src/parse/psy.rs
+++ b/src/parse/psy.rs
@@ -72,7 +72,8 @@ pub fn parse_scene<'a>(arena: &'a MemArena,
     let world = parse_world(arena, tree.iter_children_with_type("World").nth(0).unwrap())?;
 
     // Parse root scene assembly
-    let assembly = parse_assembly(tree.iter_children_with_type("Assembly").nth(0).unwrap())?;
+    let assembly = parse_assembly(arena,
+                                  tree.iter_children_with_type("Assembly").nth(0).unwrap())?;
 
     // Put scene together
     let scene_name = if let &DataTree::Internal { ident, .. } = tree {
diff --git a/src/parse/psy_assembly.rs b/src/parse/psy_assembly.rs
index ae06a47..95562ef 100644
--- a/src/parse/psy_assembly.rs
+++ b/src/parse/psy_assembly.rs
@@ -2,6 +2,8 @@
 
 use std::result::Result;
 
+use mem_arena::MemArena;
+
 use scene::{Assembly, AssemblyBuilder, Object};
 
 use super::DataTree;
@@ -10,8 +12,10 @@ use super::psy_mesh_surface::parse_mesh_surface;
 use super::psy::{parse_matrix, PsyParseError};
 
 
-pub fn parse_assembly(tree: &DataTree) -> Result<Assembly, PsyParseError> {
-    let mut builder = AssemblyBuilder::new();
+pub fn parse_assembly<'a>(arena: &'a MemArena,
+                          tree: &'a DataTree)
+                          -> Result<Assembly<'a>, PsyParseError> {
+    let mut builder = AssemblyBuilder::new(arena);
 
     if tree.is_internal() {
         for child in tree.iter_children() {
@@ -19,7 +23,7 @@ pub fn parse_assembly(tree: &DataTree) -> Result<Assembly, PsyParseError> {
                 // Sub-Assembly
                 "Assembly" => {
                     if let &DataTree::Internal { ident: Some(ident), .. } = child {
-                        builder.add_assembly(ident, parse_assembly(&child)?);
+                        builder.add_assembly(ident, parse_assembly(arena, &child)?);
                     } else {
                         // TODO: error condition of some kind, because no ident
                         panic!();
diff --git a/src/scene/assembly.rs b/src/scene/assembly.rs
index d7572e8..61eb8a2 100644
--- a/src/scene/assembly.rs
+++ b/src/scene/assembly.rs
@@ -1,5 +1,7 @@
 use std::collections::HashMap;
 
+use mem_arena::MemArena;
+
 use accel::{LightAccel, LightTree};
 use accel::BVH;
 use bbox::{BBox, transform_bbox_slice_from};
@@ -13,17 +15,17 @@ use transform_stack::TransformStack;
 
 
 #[derive(Debug)]
-pub struct Assembly {
+pub struct Assembly<'a> {
     // Instance list
-    pub instances: Vec<Instance>,
-    pub light_instances: Vec<Instance>,
-    pub xforms: Vec<Matrix4x4>,
+    pub instances: &'a [Instance],
+    pub light_instances: &'a [Instance],
+    pub xforms: &'a [Matrix4x4],
 
     // Object list
     pub objects: Vec<Object>,
 
     // Assembly list
-    pub assemblies: Vec<Assembly>,
+    pub assemblies: Vec<Assembly<'a>>,
 
     // Object accel
     pub object_accel: BVH,
@@ -32,7 +34,7 @@ pub struct Assembly {
     pub light_accel: LightTree,
 }
 
-impl Assembly {
+impl<'a> Assembly<'a> {
     // Returns (light_color, shadow_vector, pdf, selection_pdf)
     pub fn sample_lights(&self,
                          xform_stack: &mut TransformStack,
@@ -119,15 +121,17 @@ impl Assembly {
     }
 }
 
-impl Boundable for Assembly {
-    fn bounds<'a>(&'a self) -> &'a [BBox] {
+impl<'a> Boundable for Assembly<'a> {
+    fn bounds<'b>(&'b self) -> &'b [BBox] {
         self.object_accel.bounds()
     }
 }
 
 
 #[derive(Debug)]
-pub struct AssemblyBuilder {
+pub struct AssemblyBuilder<'a> {
+    arena: &'a MemArena,
+
     // Instance list
     instances: Vec<Instance>,
     xforms: Vec<Matrix4x4>,
@@ -137,14 +141,15 @@ pub struct AssemblyBuilder {
     object_map: HashMap<String, usize>, // map Name -> Index
 
     // Assembly list
-    assemblies: Vec<Assembly>,
+    assemblies: Vec<Assembly<'a>>,
     assembly_map: HashMap<String, usize>, // map Name -> Index
 }
 
 
-impl AssemblyBuilder {
-    pub fn new() -> AssemblyBuilder {
+impl<'a> AssemblyBuilder<'a> {
+    pub fn new(arena: &'a MemArena) -> AssemblyBuilder<'a> {
         AssemblyBuilder {
+            arena: arena,
             instances: Vec::new(),
             xforms: Vec::new(),
             objects: Vec::new(),
@@ -165,7 +170,7 @@ impl AssemblyBuilder {
         self.objects.push(obj);
     }
 
-    pub fn add_assembly(&mut self, name: &str, asmb: Assembly) {
+    pub fn add_assembly(&mut self, name: &str, asmb: Assembly<'a>) {
         // Make sure the name hasn't already been used.
         if self.name_exists(name) {
             panic!("Attempted to add assembly to another assembly with a name that already \
@@ -221,13 +226,7 @@ impl AssemblyBuilder {
         self.object_map.contains_key(name) || self.assembly_map.contains_key(name)
     }
 
-    pub fn build(mut self) -> Assembly {
-        // Shrink storage to minimum.
-        self.instances.shrink_to_fit();
-        self.xforms.shrink_to_fit();
-        self.objects.shrink_to_fit();
-        self.assemblies.shrink_to_fit();
-
+    pub fn build(mut self) -> Assembly<'a> {
         // Calculate instance bounds, used for building object accel and light accel.
         let (bis, bbs) = self.instance_bounds();
 
@@ -276,9 +275,9 @@ impl AssemblyBuilder {
         });
 
         Assembly {
-            instances: self.instances,
-            light_instances: light_instances,
-            xforms: self.xforms,
+            instances: self.arena.copy_slice(&self.instances),
+            light_instances: self.arena.copy_slice(&light_instances),
+            xforms: self.arena.copy_slice(&self.xforms),
             objects: self.objects,
             assemblies: self.assemblies,
             object_accel: object_accel,
diff --git a/src/scene/scene.rs b/src/scene/scene.rs
index ad8f4a6..48fb497 100644
--- a/src/scene/scene.rs
+++ b/src/scene/scene.rs
@@ -15,7 +15,7 @@ pub struct Scene<'a> {
     pub name: Option<String>,
     pub camera: Camera<'a>,
     pub world: World<'a>,
-    pub root: Assembly,
+    pub root: Assembly<'a>,
 }
 
 impl<'a> Scene<'a> {
diff --git a/src/tracer.rs b/src/tracer.rs
index c466354..592224d 100644
--- a/src/tracer.rs
+++ b/src/tracer.rs
@@ -36,7 +36,7 @@ impl<'a> Tracer<'a> {
 }
 
 struct TracerInner<'a> {
-    root: &'a Assembly,
+    root: &'a Assembly<'a>,
     xform_stack: TransformStack,
     isects: Vec<SurfaceIntersection>,
 }