diff --git a/zigrtiow/src/hittable.zig b/zigrtiow/src/hittable.zig
new file mode 100644
index 0000000..d009b3f
--- /dev/null
+++ b/zigrtiow/src/hittable.zig
@@ -0,0 +1,36 @@
+const vec = @import("./vec.zig");
+const ray = @import("./ray.zig");
+
+const Sphere = @import("./sphere.zig").Sphere;
+const HittableList = @import("./hittable_list.zig").HittableList;
+
+const Vec3 = vec.Vec3;
+const Point3 = vec.Point3;
+const Ray = ray.Ray;
+
+pub const Hittable = union(enum) {
+    sphere: Sphere,
+    hittable_list: HittableList,
+
+    pub fn hit(hittable: Hittable, r: Ray, t_min: f32, t_max: f32) ?HitRecord {
+        return switch (hittable) {
+            .sphere => |sphere| sphere.hit(r, t_min, t_max),
+            .hittable_list => |hittable_list| hittable_list.hit(r, t_min, t_max),
+        };
+    }
+};
+
+pub const HitRecord = struct {
+    p: Point3,
+    normal: Vec3,
+    t: f32,
+    front_face: bool,
+
+    pub fn set_face_normal(hr: *HitRecord, r: Ray, outward_normal: Vec3) void {
+        hr.front_face = r.direction().dot(outward_normal) < 0;
+        hr.normal = if (hr.front_face)
+            outward_normal
+        else
+            outward_normal.scale(-1);
+    }
+};
diff --git a/zigrtiow/src/hittable_list.zig b/zigrtiow/src/hittable_list.zig
new file mode 100644
index 0000000..a5f5709
--- /dev/null
+++ b/zigrtiow/src/hittable_list.zig
@@ -0,0 +1,35 @@
+const std = @import("std");
+const ray = @import("./ray.zig");
+const hittable = @import("./hittable.zig");
+
+const info = std.log.info;
+const ArrayList = std.ArrayList;
+const Ray = ray.Ray;
+const HitRecord = hittable.HitRecord;
+const Hittable = hittable.Hittable;
+
+pub const HittableList = struct {
+    objects: ArrayList(Hittable),
+
+    pub fn init() HittableList {
+        const allocator = std.heap.page_allocator;
+
+        return HittableList{
+            .objects = ArrayList(Hittable).init(allocator),
+        };
+    }
+    pub fn hit(hittable_list: HittableList, r: Ray, t_min: f32, t_max: f32) ?HitRecord {
+        var temp_rec: ?HitRecord = null;
+        var closest_so_far = t_max;
+        for (hittable_list.objects.items) |object| {
+            if (object.hit(r, t_min, closest_so_far)) |rec| {
+                temp_rec = rec;
+                closest_so_far = rec.t;
+            }
+        }
+        return temp_rec;
+    }
+    pub fn add(hittable_list: *HittableList, object: Hittable) anyerror!void {
+        try hittable_list.objects.append(object);
+    }
+};
diff --git a/zigrtiow/src/main.zig b/zigrtiow/src/main.zig
index 82d30fc..72dace7 100644
--- a/zigrtiow/src/main.zig
+++ b/zigrtiow/src/main.zig
@@ -2,27 +2,24 @@ const std = @import("std");
 const color = @import("./color.zig");
 const ray = @import("./ray.zig");
 const vec = @import("./vec.zig");
+const sphere = @import("./sphere.zig");
+const hittable = @import("./hittable.zig");
+const hittable_list = @import("./hittable_list.zig");
 
 const Color = vec.Color;
-const Vec3 = vec.Vec3;
+const Hittable = hittable.Hittable;
+const HittableList = hittable_list.HittableList;
 const Point3 = vec.Point3;
+const Ray = ray.Ray;
+const Sphere = sphere.Sphere;
+const Vec3 = vec.Vec3;
 const info = std.log.info;
 const write_color = color.write_color;
-const Ray = ray.Ray;
 
-fn hit_sphere(center: Point3, radius: f32, r: Ray) f32 {
-    const oc = r.origin().sub(center);
-    const a = r.direction().length_squared();
-    const half_b = Vec3.dot(oc, r.direction());
-    const c = oc.length_squared() - radius * radius;
-    const discriminant = half_b * half_b - a * c;
-    return if (discriminant < 0) -1 else (-half_b - @sqrt(discriminant)) / a;
-}
-
-fn ray_color(r: Ray) Color {
-    var t = hit_sphere(Point3.init(0, 0, -1), 0.5, r);
-    if (t > 0) {
-        const n = r.at(t).sub(Vec3.init(0, 0, -1)).unit();
+fn ray_color(r: Ray, world: Hittable) Color {
+    var hit = world.hit(r, 0, std.math.inf(f32));
+    if (hit) |rec| {
+        const n = rec.normal;
         return Color.init(
             n.x() + 1,
             n.y() + 1,
@@ -30,7 +27,7 @@ fn ray_color(r: Ray) Color {
         ).scale(0.5);
     }
     var unit_direction = r.direction().unit();
-    t = 0.5 * (unit_direction.y() + 1);
+    const t = 0.5 * (unit_direction.y() + 1);
     return Color.init(1, 1, 1).scale(1 - t).add(Color.init(0.5, 0.7, 1.0).scale(t));
 }
 
@@ -40,6 +37,11 @@ pub fn main() anyerror!void {
     const image_width = 400;
     const image_height = @floatToInt(isize, @intToFloat(f32, image_width) / aspect_ratio);
 
+    // World
+    var world = HittableList.init();
+    try world.add(Hittable{ .sphere = Sphere.init(Point3.init(0, 0, -1), 0.5) });
+    try world.add(Hittable{ .sphere = Sphere.init(Point3.init(0, -100.5, -1), 100) });
+
     // Camera
     const viewport_height = 2;
     const viewport_width = aspect_ratio * viewport_height;
@@ -64,7 +66,7 @@ pub fn main() anyerror!void {
             const u = @intToFloat(f32, i) / @intToFloat(f32, image_width - 1);
             const v = @intToFloat(f32, j) / @intToFloat(f32, image_height - 1);
             const r = Ray.init(origin, lower_left_corner.add(horizontal.scale(u)).add(vertical.scale(v)).sub(origin));
-            const pixel_color = ray_color(r);
+            const pixel_color = ray_color(r, Hittable{ .hittable_list = world });
             try write_color(pixel_color);
         }
     }
diff --git a/zigrtiow/src/sphere.zig b/zigrtiow/src/sphere.zig
new file mode 100644
index 0000000..81eaa23
--- /dev/null
+++ b/zigrtiow/src/sphere.zig
@@ -0,0 +1,51 @@
+const vec = @import("./vec.zig");
+const ray = @import("./ray.zig");
+const hittable = @import("./hittable.zig");
+
+const Vec3 = vec.Vec3;
+const Point3 = vec.Point3;
+const Ray = ray.Ray;
+const HitRecord = hittable.HitRecord;
+
+pub const Sphere = struct {
+    center: Point3,
+    radius: f32,
+
+    pub fn init(center: Point3, radius: f32) Sphere {
+        return Sphere{
+            .center = center,
+            .radius = radius,
+        };
+    }
+    pub fn hit(sphere: Sphere, r: Ray, t_min: f32, t_max: f32) ?HitRecord {
+        const center = sphere.center;
+        const radius = sphere.radius;
+        const oc = r.origin().sub(center);
+        const a = r.direction().length_squared();
+        const half_b = Vec3.dot(oc, r.direction());
+        const c = oc.length_squared() - radius * radius;
+
+        const discriminant = half_b * half_b - a * c;
+        if (discriminant < 0) return null;
+
+        const sqrtd = @sqrt(discriminant);
+
+        // Find the nearest root that lies in the acceptable range.
+        var root = (-half_b - sqrtd) / a;
+
+        if ((root < t_min) or (t_max < root)) {
+            root = (-half_b + sqrtd) / a;
+            if ((root < t_min) or (t_max < root)) return null;
+        }
+        const p = r.at(root);
+        const outward_normal = p.sub(center).scale(1 / radius);
+        var hr = HitRecord{
+            .t = root,
+            .p = p,
+            .normal = Vec3.init(0, 0, 0),
+            .front_face = false,
+        };
+        hr.set_face_normal(r, outward_normal);
+        return hr;
+    }
+};