diff --git a/rtiow/src/bin/tracer_multiple_spheres.rs b/rtiow/src/bin/tracer_multiple_spheres.rs
new file mode 100644
index 0000000..132fcce
--- /dev/null
+++ b/rtiow/src/bin/tracer_multiple_spheres.rs
@@ -0,0 +1,46 @@
+extern crate rtiow;
+
+use rtiow::hitable::Hit;
+use rtiow::hitable_list::HitableList;
+use rtiow::ray::Ray;
+use rtiow::sphere::Sphere;
+use rtiow::vec3::Vec3;
+
+fn color(r: Ray, world: &Hit) -> Vec3 {
+    if let Some(rec) = world.hit(r, 0., std::f32::MAX) {
+        return (rec.normal + 1.) * 0.5;
+    }
+
+    // No hit, choose color from background.
+    let unit_direction = r.direction().unit_vector();
+    let t = 0.5 * (unit_direction.y + 1.);
+    Vec3::new(1., 1., 1.) * (1. - t) + Vec3::new(0.5, 0.7, 1.) * t
+}
+
+fn main() -> Result<(), std::io::Error> {
+    let nx = 200;
+    let ny = 100;
+    println!("P3\n{} {}\n255", nx, ny);
+    let lower_left_corner = Vec3::new(-2., -1., -1.);
+    let horizontal = Vec3::new(4., 0., 0.);
+    let vertical = Vec3::new(0., 2., 0.);
+    let origin: Vec3 = Default::default();
+    let objects = vec![
+        Sphere::new(Vec3::new(0., 0., -1.), 0.5),
+        Sphere::new(Vec3::new(0., -100.5, -1.), 100.),
+    ];
+    let world = HitableList::new(objects.iter().map(|o| o).collect());
+    for j in (0..ny).rev() {
+        for i in 0..nx {
+            let u = i as f32 / nx as f32;
+            let v = j as f32 / ny as f32;
+            let r = Ray::new(origin, lower_left_corner + horizontal * u + vertical * v);
+            let col = color(r, &world);
+            let ir = (255.99 * col[0]) as u32;
+            let ig = (255.99 * col[1]) as u32;
+            let ib = (255.99 * col[2]) as u32;
+            println!("{} {} {}", ir, ig, ib);
+        }
+    }
+    Ok(())
+}
diff --git a/rtiow/src/hitable.rs b/rtiow/src/hitable.rs
new file mode 100644
index 0000000..86295ac
--- /dev/null
+++ b/rtiow/src/hitable.rs
@@ -0,0 +1,13 @@
+use ray::Ray;
+use vec3::Vec3;
+
+#[derive(Copy, Clone)]
+pub struct HitRecord {
+    pub t: f32,
+    pub p: Vec3,
+    pub normal: Vec3,
+}
+
+pub trait Hit {
+    fn hit(&self, r: Ray, t_min: f32, t_max: f32) -> Option<HitRecord>;
+}
diff --git a/rtiow/src/hitable_list.rs b/rtiow/src/hitable_list.rs
new file mode 100644
index 0000000..11a271d
--- /dev/null
+++ b/rtiow/src/hitable_list.rs
@@ -0,0 +1,37 @@
+use hitable::Hit;
+use hitable::HitRecord;
+use ray::Ray;
+
+#[derive(Default)]
+pub struct HitableList<'a, H>
+where
+    H: Hit + 'a,
+{
+    list: Vec<&'a H>,
+}
+
+impl<'a, H> HitableList<'a, H>
+where
+    H: Hit,
+{
+    pub fn new(list: Vec<&'a H>) -> HitableList<H> {
+        HitableList { list }
+    }
+}
+
+impl<'a, H> Hit for HitableList<'a, H>
+where
+    H: Hit,
+{
+    fn hit(&self, r: Ray, t_min: f32, t_max: f32) -> Option<HitRecord> {
+        let mut min_hit = None;
+        let mut closest_so_far = t_max;
+        for hitable in &self.list {
+            if let Some(h) = hitable.hit(r, t_min, closest_so_far) {
+                closest_so_far = h.t;
+                min_hit = Some(h);
+            }
+        }
+        min_hit
+    }
+}
diff --git a/rtiow/src/lib.rs b/rtiow/src/lib.rs
index 1d07923..d1a66f2 100644
--- a/rtiow/src/lib.rs
+++ b/rtiow/src/lib.rs
@@ -1,2 +1,5 @@
+pub mod hitable;
+pub mod hitable_list;
 pub mod ray;
+pub mod sphere;
 pub mod vec3;
diff --git a/rtiow/src/sphere.rs b/rtiow/src/sphere.rs
new file mode 100644
index 0000000..d0a900c
--- /dev/null
+++ b/rtiow/src/sphere.rs
@@ -0,0 +1,48 @@
+use hitable::Hit;
+use hitable::HitRecord;
+use ray::Ray;
+use vec3::dot;
+use vec3::Vec3;
+
+#[derive(Copy, Clone, Default)]
+pub struct Sphere {
+    center: Vec3,
+    radius: f32,
+}
+
+impl Sphere {
+    pub fn new(center: Vec3, radius: f32) -> Sphere {
+        Sphere { center, radius }
+    }
+}
+
+impl Hit for Sphere {
+    fn hit(&self, r: Ray, t_min: f32, t_max: f32) -> Option<HitRecord> {
+        let oc = r.origin() - self.center;
+        let a = dot(r.direction(), r.direction());
+        let b = dot(oc, r.direction());
+        let c = dot(oc, oc) - self.radius * self.radius;
+        let discriminant = b * b - a * c;
+        if discriminant > 0. {
+            let temp = (-b - (b * b - a * c).sqrt()) / a;
+            if temp < t_max && temp > t_min {
+                let p = r.point_at_parameter(temp);
+                return Some(HitRecord {
+                    t: temp,
+                    p,
+                    normal: (p - self.center) / self.radius,
+                });
+            }
+            let temp = (-b + (b * b - a * c).sqrt()) / a;
+            if temp < t_max && temp > t_min {
+                let p = r.point_at_parameter(temp);
+                return Some(HitRecord {
+                    t: temp,
+                    p,
+                    normal: (p - self.center) / self.radius,
+                });
+            }
+        }
+        None
+    }
+}