diff --git a/Quik/CommandQueue/CommandEngine.cs b/Quik/CommandQueue/CommandEngine.cs
new file mode 100644
index 0000000..5f91ee9
--- /dev/null
+++ b/Quik/CommandQueue/CommandEngine.cs
@@ -0,0 +1,163 @@
+using System;
+using System.Collections.Generic;
+
+namespace Quik.CommandQueue
+{
+    public class CommandEngine
+    {
+        private int _zIndex = 0;
+        private readonly Stack<int> _zStack = new Stack<int>();
+        public int ZIndex => _zIndex;
+
+        private QuikRectangle _viewport;
+        private readonly Stack<QuikRectangle> _viewportStack = new Stack<QuikRectangle>();
+        private readonly Stack<object>  _matrixStack = new Stack<object>();
+
+        private Command _customCommandBase = Command.CustomCommandBase;
+        private readonly List<QuikCommandHandler> _customCommands = new List<QuikCommandHandler>();
+
+        public QuikRectangle Viewport { get; }
+
+        // TODO: Make a real matrix class.
+        public float[] ActiveTransforms { get; }
+
+        public object ActiveStyle { get; }
+
+        protected CommandEngine()
+        {
+            Reset();
+        }
+
+        public Command RegisterCustomCommand(QuikCommandHandler handler)
+        {
+            Command id = _customCommandBase++;
+            _customCommands.Insert(id - Command.CustomCommandBase, handler);
+            return id;
+        }
+
+        public void ProcessCommands(QuikRectangle bounds, CommandQueue queue)
+        {
+            if (!queue.Peek().IsCommand)
+                throw new ArgumentException("The first element in the queue must be a command frame.");
+
+            Reset();
+
+            _viewport = bounds;
+            _viewportStack.Push(_viewport);
+
+            Frame frame;
+            while (queue.TryDequeue(out frame))
+            {
+                Command cmd = (Command)frame;
+                switch (cmd)
+                {
+                default:
+                    if (cmd > Command.CustomCommandBase)
+                    {
+                        _customCommands[cmd - Command.CustomCommandBase].Invoke(this, queue);
+                    }
+                    else
+                    {
+                        ChildProcessCommand(cmd, queue);
+                    }
+                    break;
+
+                case Command.ConditionalBegin:  ConditionalHandler(queue);       break;
+                case Command.ConditionalEnd:    /* nop */                        break;
+
+                case Command.Invoke:
+                    queue.Dequeue().As<QuikCommandHandler>().Invoke(this, queue);
+                    break;
+                case Command.PushViewport:
+                    _viewportStack.Push(_viewport);
+                    break;
+                case Command.IntersectViewport:
+                    _viewport = QuikRectangle.Intersect((QuikRectangle)queue.Dequeue(), _viewport);
+                    break;
+                case Command.StoreViewport:
+                    _viewport = (QuikRectangle)queue.Dequeue();
+                    break;
+                case Command.PopViewport:
+                    _viewport = _viewportStack.TryPop(out QuikRectangle viewport) ? viewport : bounds;
+                    break;
+                case Command.PushZ:
+                    _zStack.Push(_zIndex);
+                    break;
+                case Command.IncrementZ:
+                    _zIndex++;
+                    break;
+                case Command.AddZ:
+                    _zIndex += (int)queue.Dequeue();
+                    break;
+                case Command.StoreZ:
+                    _zIndex = (int)queue.Dequeue();
+                    break;
+                case Command.DecrementZ:
+                    _zIndex--;
+                    break;
+                case Command.PopZ:
+                    _zIndex = _zStack.TryPop(out int zindex) ? zindex : 0;
+                    break;
+                }
+            }
+        }
+
+        protected virtual void ChildProcessCommand(Command name, CommandQueue queue)
+        {
+        }
+
+        private void Reset()
+        {
+            _zIndex = 0;
+            _zStack.Clear();
+
+            _viewport = new QuikRectangle(float.MaxValue, float.MaxValue, float.MinValue, float.MinValue);
+            _viewportStack.Clear();
+
+            _matrixStack.Clear();
+            _matrixStack.Push(null);    // TODO: replace with identity matrix.
+        }
+
+        private void ConditionalHandler(CommandQueue queue)
+        {
+            Frame frame = queue.Dequeue();
+
+            if (
+                frame.IsInteger && (int)frame != 0 ||
+                frame.As<Func<bool>>().Invoke())
+            {
+                // Take this branch.
+                return;
+            }
+
+            // Skip this branch.
+            int depth = 1;
+            while (queue.TryPeek(out frame))
+            {
+                if (!frame.IsCommand)
+                {
+                    queue.Dequeue();
+                    continue;
+                }
+
+                switch ((Command)frame)
+                {
+                case Command.ConditionalBegin:
+                    // Increment conditional depth.
+                    depth++;
+                    break;
+                case Command.ConditionalEnd:
+                    // Decrement condional depth, exit if zero.
+                    if (--depth == 0)
+                    {
+                        queue.Dequeue();
+                        return;
+                    }
+                    break;
+                }
+
+                queue.Dequeue();
+            }
+        }
+    }
+}
\ No newline at end of file
diff --git a/Quik/CommandQueue/CommandHandler.cs b/Quik/CommandQueue/CommandHandler.cs
index 01e2b67..50dc6f9 100644
--- a/Quik/CommandQueue/CommandHandler.cs
+++ b/Quik/CommandQueue/CommandHandler.cs
@@ -4,5 +4,5 @@ namespace Quik.CommandQueue
     /// A delegate for a QUIK command.
     /// </summary>
     /// <param name="stack">The current stack.</param>
-    public delegate void QuikCommandHandler(object state);
+    public delegate void QuikCommandHandler(CommandEngine state, CommandQueue queue);
 }
\ No newline at end of file