diff --git a/zigrtiow/src/main.zig b/zigrtiow/src/main.zig
index 275393a..8b56961 100644
--- a/zigrtiow/src/main.zig
+++ b/zigrtiow/src/main.zig
@@ -8,6 +8,8 @@ const hittable = @import("./hittable.zig");
 const hittable_list = @import("./hittable_list.zig");
 const material = @import("./material.zig");
 
+const Thread = std.Thread;
+const Queue = std.atomic.Queue;
 const Camera = camera.Camera;
 const Color = vec.Color;
 const Hittable = hittable.Hittable;
@@ -38,6 +40,43 @@ fn ray_color(r: Ray, world: Hittable, depth: isize) Color {
     return Color.init(1, 1, 1).scale(1 - t).add(Color.init(0.5, 0.7, 1.0).scale(t));
 }
 
+const Task = struct {
+    cam: Camera,
+    samples_per_pixel: usize,
+    max_depth: isize,
+    row_idx: usize,
+    image_height: isize,
+    image_width: isize,
+    row_pixels: []Color,
+    world: Hittable,
+};
+
+fn render_row(q: *Queue(Task)) void {
+    var prng = std.rand.DefaultPrng.init(0);
+    const rand = prng.random();
+    while (true) {
+        if (q.get()) |node| {
+            const task = node.data;
+            info("Rendering row: {}", .{task.row_idx});
+            const j = task.row_idx;
+            var i: usize = 0;
+            while (i < task.image_width) : (i += 1) {
+                var pixel_color = Color.init(0, 0, 0);
+                var s: isize = 0;
+                while (s < task.samples_per_pixel) : (s += 1) {
+                    const u = (@intToFloat(f32, i) + rand.float(f32)) / @intToFloat(f32, task.image_width - 1);
+                    const v = (@intToFloat(f32, j) + rand.float(f32)) / @intToFloat(f32, task.image_height - 1);
+                    const r = task.cam.get_ray(u, v);
+                    pixel_color = pixel_color.add(ray_color(r, task.world, task.max_depth));
+                }
+                task.row_pixels[i] = pixel_color;
+            }
+        } else {
+            return;
+        }
+    }
+}
+
 pub fn main() anyerror!void {
     // Image
     const aspect_ratio: f32 = 16.0 / 9.0;
@@ -64,27 +103,56 @@ pub fn main() anyerror!void {
     const cam = Camera.init();
 
     // Render
+    const Node = Queue(Task).Node;
+    const allocator = std.heap.page_allocator;
+    const cpus = try Thread.getCpuCount();
+    var pixels: [image_width * image_height]Color = undefined;
+    var q = Queue(Task).init();
+    var threads: std.ArrayList(std.Thread) = std.ArrayList(std.Thread).init(allocator);
+    var row: usize = 0;
+    while (row < image_height) : (row += 1) {
+        const node = try allocator.create(Node);
+        const start: usize = row * image_width;
+        const end: usize = (row + 1) * image_width;
+        node.* = .{
+            .prev = undefined,
+            .next = undefined,
+            .data = Task{
+                .cam = cam,
+                .samples_per_pixel = samples_per_pixel,
+                .max_depth = max_depth,
+                .row_idx = row,
+                .image_width = image_width,
+                .image_height = image_height,
+                .row_pixels = pixels[start..end],
+                .world = world,
+            },
+        };
+        q.put(node);
+    }
+
+    var t: usize = 0;
+    while (t < cpus) : (t += 1) {
+        try threads.append(try Thread.spawn(.{}, render_row, .{&q}));
+    }
+
+    // Wait for all threads to finish.
+    for (threads.items) |thread| {
+        Thread.join(thread);
+    }
+
     const stdout = std.io.getStdOut();
 
     info("Writing image {d}x{d}", .{ image_width, image_height });
     try stdout.writer().print("P3\n{d} {d}\n255\n", .{ image_width, image_height });
 
-    var prng = std.rand.DefaultPrng.init(0);
-    const rand = prng.random();
-    var j: isize = image_height - 1;
-    while (j >= 0) : (j -= 1) {
-        info("Scanlines remaining: {d}", .{j});
-
-        var i: isize = 0;
+    var j: usize = 0;
+    while (j < image_height) : (j += 1) {
+        var i: usize = 0;
         while (i < image_width) : (i += 1) {
-            var pixel_color = Color.init(0, 0, 0);
-            var s: isize = 0;
-            while (s < samples_per_pixel) : (s += 1) {
-                const u = (@intToFloat(f32, i) + rand.float(f32)) / @intToFloat(f32, image_width - 1);
-                const v = (@intToFloat(f32, j) + rand.float(f32)) / @intToFloat(f32, image_height - 1);
-                const r = cam.get_ray(u, v);
-                pixel_color = pixel_color.add(ray_color(r, world, max_depth));
-            }
+            // Flip image upside down.
+            const j_idx = @as(usize, image_height) - j - 1;
+            const pixel_color = pixels[i + j_idx * image_width];
             try write_color(pixel_color, samples_per_pixel);
         }
     }