diff --git a/rtiow/src/renderer.rs b/rtiow/src/renderer.rs
index 72ac4e9..a1c7af0 100644
--- a/rtiow/src/renderer.rs
+++ b/rtiow/src/renderer.rs
@@ -119,6 +119,9 @@ pub struct Opt {
     /// Image height
     #[structopt(short = "h", long = "height", default_value = "1024")]
     pub height: usize,
+    /// Number of threads
+    #[structopt(short = "t", long = "num_threads")]
+    pub num_threads: Option<usize>,
     /// Sub-samples per pixel
     #[structopt(short = "s", long = "subsample", default_value = "8")]
     pub subsamples: usize,
@@ -139,6 +142,7 @@ pub struct Opt {
 }
 
 pub fn opt_hash(opt: &Opt) -> String {
+    // TODO(wathiede): add threads.
     format!(
         "w:{}-h:{}-s:{}-pprof:{}-model:{}-use_accel:{}-{}",
         opt.width,
@@ -155,6 +159,7 @@ pub struct Scene {
     pub world: Box<Hit>,
     pub camera: Camera,
     pub subsamples: usize,
+    pub num_threads: Option<usize>,
     pub width: usize,
     pub height: usize,
     pub global_illumination: bool,
@@ -269,13 +274,13 @@ fn render_worker(
 }
 
 pub fn render(scene: Scene, output_dir: &Path) -> std::result::Result<(), std::io::Error> {
-    let cpus = num_cpus::get();
-    let (pixel_req_tx, pixel_req_rx) = channel::bounded(2 * cpus);
-    let (pixel_resp_tx, pixel_resp_rx) = channel::bounded(2 * cpus);
+    let num_threads = scene.num_threads.unwrap_or_else(num_cpus::get);
+    let (pixel_req_tx, pixel_req_rx) = channel::bounded(2 * num_threads);
+    let (pixel_resp_tx, pixel_resp_rx) = channel::bounded(2 * num_threads);
 
     let scene = sync::Arc::new(scene);
-    println!("Creating {} render threads", cpus);
-    for i in 0..cpus {
+    println!("Creating {} render threads", num_threads);
+    for i in 0..num_threads {
         let s = sync::Arc::clone(&scene);
         let pixel_req_rx = pixel_req_rx.clone();
         let pixel_resp_tx = pixel_resp_tx.clone();
diff --git a/rtiow/src/scenes/bench.rs b/rtiow/src/scenes/bench.rs
index 50475f2..4f2cd80 100644
--- a/rtiow/src/scenes/bench.rs
+++ b/rtiow/src/scenes/bench.rs
@@ -69,6 +69,7 @@ pub fn new(opt: &Opt) -> Scene {
         camera,
         world,
         subsamples: opt.subsamples,
+        num_threads: opt.num_threads,
         width: opt.width,
         height: opt.height,
         global_illumination: true,
diff --git a/rtiow/src/scenes/book.rs b/rtiow/src/scenes/book.rs
index 6c5f912..db5ce51 100644
--- a/rtiow/src/scenes/book.rs
+++ b/rtiow/src/scenes/book.rs
@@ -51,6 +51,7 @@ pub fn new(opt: &Opt) -> Scene {
         camera,
         world,
         subsamples: opt.subsamples,
+        num_threads: opt.num_threads,
         width: opt.width,
         height: opt.height,
         global_illumination: true,
diff --git a/rtiow/src/scenes/bvh.rs b/rtiow/src/scenes/bvh.rs
index 8ffc228..b23cec5 100644
--- a/rtiow/src/scenes/bvh.rs
+++ b/rtiow/src/scenes/bvh.rs
@@ -63,6 +63,7 @@ pub fn new(opt: &Opt) -> Scene {
         camera,
         world,
         subsamples: opt.subsamples,
+        num_threads: opt.num_threads,
         width: opt.width,
         height: opt.height,
         global_illumination: true,
diff --git a/rtiow/src/scenes/cornell_box.rs b/rtiow/src/scenes/cornell_box.rs
index 2b574ac..42fd2f2 100644
--- a/rtiow/src/scenes/cornell_box.rs
+++ b/rtiow/src/scenes/cornell_box.rs
@@ -130,6 +130,7 @@ pub fn new(opt: &Opt) -> Scene {
         camera,
         world,
         subsamples: opt.subsamples,
+        num_threads: opt.num_threads,
         width: opt.width,
         height: opt.height,
         global_illumination: false,
diff --git a/rtiow/src/scenes/cornell_smoke.rs b/rtiow/src/scenes/cornell_smoke.rs
index a54e9f2..f7f0765 100644
--- a/rtiow/src/scenes/cornell_smoke.rs
+++ b/rtiow/src/scenes/cornell_smoke.rs
@@ -115,6 +115,7 @@ pub fn new(opt: &Opt) -> Scene {
         camera,
         world,
         subsamples: opt.subsamples,
+        num_threads: opt.num_threads,
         width: opt.width,
         height: opt.height,
         global_illumination: false,
diff --git a/rtiow/src/scenes/final_scene.rs b/rtiow/src/scenes/final_scene.rs
index e7d8c02..e3e5d33 100644
--- a/rtiow/src/scenes/final_scene.rs
+++ b/rtiow/src/scenes/final_scene.rs
@@ -166,6 +166,7 @@ pub fn new(opt: &Opt) -> Scene {
         camera,
         world: Box::new(HitableList::new(list)),
         subsamples: opt.subsamples,
+        num_threads: opt.num_threads,
         width: opt.width,
         height: opt.height,
         global_illumination: false,
diff --git a/rtiow/src/scenes/mandelbrot.rs b/rtiow/src/scenes/mandelbrot.rs
index 7237222..1783b6d 100644
--- a/rtiow/src/scenes/mandelbrot.rs
+++ b/rtiow/src/scenes/mandelbrot.rs
@@ -144,6 +144,7 @@ pub fn new(opt: &Opt) -> Scene {
         camera,
         world,
         subsamples: opt.subsamples,
+        num_threads: opt.num_threads,
         width: opt.width,
         height: opt.height,
         global_illumination: false,
diff --git a/rtiow/src/scenes/perlin_debug.rs b/rtiow/src/scenes/perlin_debug.rs
index 32091e7..b5b789e 100644
--- a/rtiow/src/scenes/perlin_debug.rs
+++ b/rtiow/src/scenes/perlin_debug.rs
@@ -65,6 +65,7 @@ pub fn new(opt: &Opt) -> Scene {
         camera,
         world,
         subsamples: opt.subsamples,
+        num_threads: opt.num_threads,
         width: opt.width,
         height: opt.height,
         global_illumination: true,
diff --git a/rtiow/src/scenes/test.rs b/rtiow/src/scenes/test.rs
index 143dcd6..461e7b1 100644
--- a/rtiow/src/scenes/test.rs
+++ b/rtiow/src/scenes/test.rs
@@ -142,6 +142,7 @@ pub fn new(opt: &Opt) -> Scene {
         camera,
         world,
         subsamples: opt.subsamples,
+        num_threads: opt.num_threads,
         width: opt.width,
         height: opt.height,
         global_illumination: false,
diff --git a/rtiow/src/scenes/tutorial.rs b/rtiow/src/scenes/tutorial.rs
index caefd56..bf34ea9 100644
--- a/rtiow/src/scenes/tutorial.rs
+++ b/rtiow/src/scenes/tutorial.rs
@@ -70,6 +70,7 @@ pub fn new(opt: &Opt) -> Scene {
         camera,
         world,
         subsamples: opt.subsamples,
+        num_threads: opt.num_threads,
         width: opt.width,
         height: opt.height,
         global_illumination: true,