diff --git a/Perspex.UnitTests/Perspex.UnitTests.csproj b/Perspex.UnitTests/Perspex.UnitTests.csproj
index 8786b08e30..854fcfd097 100644
--- a/Perspex.UnitTests/Perspex.UnitTests.csproj
+++ b/Perspex.UnitTests/Perspex.UnitTests.csproj
@@ -72,6 +72,7 @@
     <Compile Include="Properties\AssemblyInfo.cs" />
     <Compile Include="PerspexObjectTests.cs" />
     <Compile Include="StyleTests.cs" />
+    <Compile Include="Styling\ActivatorTests.cs" />
     <Compile Include="Styling\SelectorTests_Class.cs" />
     <Compile Include="Styling\SelectorTests_Id.cs" />
     <Compile Include="Styling\SelectorTests_OfType.cs" />
@@ -82,6 +83,8 @@
     <Compile Include="Styling\TestSelectors.cs" />
     <Compile Include="Styling\TestObservable.cs" />
     <Compile Include="Styling\TestControlBase.cs" />
+    <Compile Include="TestSubject.cs" />
+    <Compile Include="TestObserver.cs" />
   </ItemGroup>
   <ItemGroup>
     <ProjectReference Include="..\Perspex\Perspex.csproj">
diff --git a/Perspex.UnitTests/Styling/ActivatorTests.cs b/Perspex.UnitTests/Styling/ActivatorTests.cs
new file mode 100644
index 0000000000..e21abe7500
--- /dev/null
+++ b/Perspex.UnitTests/Styling/ActivatorTests.cs
@@ -0,0 +1,200 @@
+namespace Perspex.UnitTests.Styling
+{
+    using System;
+    using System.Collections.Generic;
+    using System.Linq;
+    using System.Reactive.Linq;
+    using System.Reactive.Subjects;
+    using System.Text;
+    using System.Threading.Tasks;
+    using Microsoft.VisualStudio.TestTools.UnitTesting;
+    using Perspex.Styling;
+    using Activator = Perspex.Styling.Activator;
+
+    [TestClass]
+    public class ActivatorTests
+    {
+        [TestMethod]
+        public void Activator_And_Should_Follow_Single_Input()
+        {
+            var inputs = new[] { new TestSubject<bool>(false) };
+            var target = new Activator(inputs, ActivatorMode.And);
+            var result = new TestObserver<bool>();
+
+            target.Subscribe(result);
+            Assert.IsFalse(result.GetValue());
+            inputs[0].OnNext(true);
+            Assert.IsTrue(result.GetValue());
+            inputs[0].OnNext(false);
+            Assert.IsFalse(result.GetValue());
+            inputs[0].OnNext(true);
+            Assert.IsTrue(result.GetValue());
+
+            Assert.AreEqual(1, inputs[0].SubscriberCount);
+        }
+
+        [TestMethod]
+        public void Activator_And_Should_AND_Multiple_Inputs()
+        {
+            var inputs = new[] 
+            { 
+                new TestSubject<bool>(false),
+                new TestSubject<bool>(false),
+                new TestSubject<bool>(true),
+            };
+            var target = new Activator(inputs, ActivatorMode.And);
+            var result = new TestObserver<bool>();
+
+            target.Subscribe(result);
+            Assert.IsFalse(result.GetValue());
+            inputs[0].OnNext(true);
+            inputs[1].OnNext(true);
+            Assert.IsTrue(result.GetValue());
+            inputs[0].OnNext(false);
+            Assert.IsFalse(result.GetValue());
+
+            Assert.AreEqual(1, inputs[0].SubscriberCount);
+            Assert.AreEqual(1, inputs[1].SubscriberCount);
+            Assert.AreEqual(1, inputs[2].SubscriberCount);
+        }
+
+        [TestMethod]
+        public void Activator_And_Should_Unsubscribe_All_When_Input_Completes_On_False()
+        {
+            var inputs = new[] 
+            { 
+                new TestSubject<bool>(false),
+                new TestSubject<bool>(false),
+                new TestSubject<bool>(true),
+            };
+            var target = new Activator(inputs, ActivatorMode.And);
+            var result = new TestObserver<bool>();
+
+            target.Subscribe(result);
+            Assert.IsFalse(result.GetValue());
+            inputs[0].OnNext(true);
+            inputs[1].OnNext(true);
+            Assert.IsTrue(result.GetValue());
+            inputs[0].OnNext(false);
+            Assert.IsFalse(result.GetValue());
+            inputs[0].OnCompleted();
+
+            Assert.AreEqual(0, inputs[0].SubscriberCount);
+            Assert.AreEqual(0, inputs[1].SubscriberCount);
+            Assert.AreEqual(0, inputs[2].SubscriberCount);
+        }
+
+        [TestMethod]
+        public void Activator_And_Should_Not_Unsubscribe_All_When_Input_Completes_On_True()
+        {
+            var inputs = new[] 
+            { 
+                new TestSubject<bool>(false),
+                new TestSubject<bool>(false),
+                new TestSubject<bool>(true),
+            };
+            var target = new Activator(inputs, ActivatorMode.And);
+            var result = new TestObserver<bool>();
+
+            target.Subscribe(result);
+            Assert.IsFalse(result.GetValue());
+            inputs[0].OnNext(true);
+            inputs[0].OnCompleted();
+
+            Assert.AreEqual(1, inputs[0].SubscriberCount);
+            Assert.AreEqual(1, inputs[1].SubscriberCount);
+            Assert.AreEqual(1, inputs[2].SubscriberCount);
+        }
+
+        [TestMethod]
+        public void Activator_Or_Should_Follow_Single_Input()
+        {
+            var inputs = new[] { new TestSubject<bool>(false) };
+            var target = new Activator(inputs, ActivatorMode.Or);
+            var result = new TestObserver<bool>();
+
+            target.Subscribe(result);
+            Assert.IsFalse(result.GetValue());
+            inputs[0].OnNext(true);
+            Assert.IsTrue(result.GetValue());
+            inputs[0].OnNext(false);
+            Assert.IsFalse(result.GetValue());
+            inputs[0].OnNext(true);
+            Assert.IsTrue(result.GetValue());
+
+            Assert.AreEqual(1, inputs[0].SubscriberCount);
+        }
+
+        [TestMethod]
+        public void Activator_Or_Should_OR_Multiple_Inputs()
+        {
+            var inputs = new[] 
+            { 
+                new TestSubject<bool>(false),
+                new TestSubject<bool>(false),
+                new TestSubject<bool>(true),
+            };
+            var target = new Activator(inputs, ActivatorMode.Or);
+            var result = new TestObserver<bool>();
+
+            target.Subscribe(result);
+            Assert.IsTrue(result.GetValue());
+            inputs[2].OnNext(false);
+            Assert.IsFalse(result.GetValue());
+            inputs[0].OnNext(true);
+            Assert.IsTrue(result.GetValue());
+
+            Assert.AreEqual(1, inputs[0].SubscriberCount);
+            Assert.AreEqual(1, inputs[1].SubscriberCount);
+            Assert.AreEqual(1, inputs[2].SubscriberCount);
+        }
+
+        [TestMethod]
+        public void Activator_Or_Should_Unsubscribe_All_When_Input_Completes_On_True()
+        {
+            var inputs = new[] 
+            { 
+                new TestSubject<bool>(false),
+                new TestSubject<bool>(false),
+                new TestSubject<bool>(true),
+            };
+            var target = new Activator(inputs, ActivatorMode.Or);
+            var result = new TestObserver<bool>();
+
+            target.Subscribe(result);
+            Assert.IsTrue(result.GetValue());
+            inputs[2].OnNext(false);
+            Assert.IsFalse(result.GetValue());
+            inputs[0].OnNext(true);
+            Assert.IsTrue(result.GetValue());
+            inputs[0].OnCompleted();
+
+            Assert.AreEqual(0, inputs[0].SubscriberCount);
+            Assert.AreEqual(0, inputs[1].SubscriberCount);
+            Assert.AreEqual(0, inputs[2].SubscriberCount);
+        }
+
+        [TestMethod]
+        public void Activator_Or_Should_Not_Unsubscribe_All_When_Input_Completes_On_False()
+        {
+            var inputs = new[] 
+            { 
+                new TestSubject<bool>(false),
+                new TestSubject<bool>(false),
+                new TestSubject<bool>(true),
+            };
+            var target = new Activator(inputs, ActivatorMode.Or);
+            var result = new TestObserver<bool>();
+
+            target.Subscribe(result);
+            Assert.IsTrue(result.GetValue());
+            inputs[2].OnNext(false);
+            Assert.IsFalse(result.GetValue());
+            inputs[2].OnCompleted();
+
+            Assert.AreEqual(1, inputs[0].SubscriberCount);
+            Assert.AreEqual(1, inputs[1].SubscriberCount);
+            Assert.AreEqual(1, inputs[2].SubscriberCount);
+        }
+    }
+}
diff --git a/Perspex.UnitTests/TestObserver.cs b/Perspex.UnitTests/TestObserver.cs
new file mode 100644
index 0000000000..aeee1d4cac
--- /dev/null
+++ b/Perspex.UnitTests/TestObserver.cs
@@ -0,0 +1,65 @@
+// -----------------------------------------------------------------------
+// <copyright file="TestObserver.cs" company="Steven Kirk">
+// Copyright 2014 MIT Licence. See licence.md for more information.
+// </copyright>
+// -----------------------------------------------------------------------
+
+namespace Perspex.UnitTests
+{
+    using System;
+
+    internal class TestObserver<T> : IObserver<T>
+    {
+        private bool hasValue;
+
+        private T value;
+
+        public bool Completed { get; private set; }
+
+        public Exception Error { get; private set; }
+
+        public T GetValue()
+        {
+            if (!this.hasValue)
+            {
+                throw new Exception("Observable provided no value.");
+            }
+
+            if (this.Completed)
+            {
+                throw new Exception("Observable completed unexpectedly.");
+            }
+
+            if (this.Error != null)
+            {
+                throw new Exception("Observable errored unexpectedly.");
+            }
+
+            this.hasValue = false;
+            return value;
+        }
+
+        public void OnCompleted()
+        {
+            this.Completed = true;
+        }
+
+        public void OnError(Exception error)
+        {
+            this.Error = error;
+        }
+
+        public void OnNext(T value)
+        {
+            if (!this.hasValue)
+            {
+                this.value = value;
+                this.hasValue = true;
+            }
+            else
+            {
+                throw new Exception("Observable pushed more than one value.");
+            }
+        }
+    }
+}
diff --git a/Perspex.UnitTests/TestSubject.cs b/Perspex.UnitTests/TestSubject.cs
new file mode 100644
index 0000000000..0a99a5df93
--- /dev/null
+++ b/Perspex.UnitTests/TestSubject.cs
@@ -0,0 +1,60 @@
+// -----------------------------------------------------------------------
+// <copyright file="TestSubject.cs" company="Steven Kirk">
+// Copyright 2014 MIT Licence. See licence.md for more information.
+// </copyright>
+// -----------------------------------------------------------------------
+
+namespace Perspex.UnitTests
+{
+    using System;
+    using System.Collections.Generic;
+    using System.Reactive.Disposables;
+
+    internal class TestSubject<T> : IObserver<T>, IObservable<T>
+    {
+        private T initial;
+
+        private List<IObserver<T>> subscribers = new List<IObserver<T>>();
+
+        public TestSubject(T initial)
+        {
+            this.initial = initial;
+        }
+
+        public int SubscriberCount
+        {
+            get { return this.subscribers.Count; }
+        }
+
+        public void OnCompleted()
+        {
+            foreach (IObserver<T> subscriber in this.subscribers.ToArray())
+            {
+                subscriber.OnCompleted();
+            }
+        }
+
+        public void OnError(Exception error)
+        {
+            foreach (IObserver<T> subscriber in this.subscribers.ToArray())
+            {
+                subscriber.OnError(error);
+            }
+        }
+
+        public void OnNext(T value)
+        {
+            foreach (IObserver<T> subscriber in this.subscribers.ToArray())
+            {
+                subscriber.OnNext(value);
+            }
+        }
+
+        public IDisposable Subscribe(IObserver<T> observer)
+        {
+            this.subscribers.Add(observer);
+            observer.OnNext(initial);
+            return Disposable.Create(() => this.subscribers.Remove(observer));
+        }
+    }
+}
diff --git a/Perspex/Styling/Activator.cs b/Perspex/Styling/Activator.cs
index 860e682a68..5b4e5b88c5 100644
--- a/Perspex/Styling/Activator.cs
+++ b/Perspex/Styling/Activator.cs
@@ -11,8 +11,16 @@ namespace Perspex.Styling
     using System.Linq;
     using System.Reactive.Disposables;
 
+    public enum ActivatorMode
+    {
+        And,
+        Or,
+    }
+
     public class Activator : IObservable<bool>
     {
+        ActivatorMode mode;
+
         List<bool> values = new List<bool>();
 
         List<IDisposable> subscriptions = new List<IDisposable>();
@@ -21,27 +29,24 @@ namespace Perspex.Styling
 
         bool last = false;
 
-        public Activator(Selector match, IStyleable control)
+        public Activator(IEnumerable<IObservable<bool>> inputs, ActivatorMode mode = ActivatorMode.And)
         {
             int i = 0;
 
-            while (match != null)
+            this.mode = mode;
+
+            foreach (IObservable<bool> input in inputs)
             {
                 int iCaptured = i;
 
-                if (match.Observable != null)
-                {
-                    this.values.Add(false);
-
-                    IDisposable subscription = match.Observable(control).Subscribe(
-                        x => this.Update(iCaptured, x),
-                        x => this.Finish(iCaptured),
-                        () => this.Finish(iCaptured));
-                    this.subscriptions.Add(subscription);
-                    ++i;
-                }
+                this.values.Add(false);
 
-                match = match.Previous;
+                IDisposable subscription = input.Subscribe(
+                    x => this.Update(iCaptured, x),
+                    x => this.Finish(iCaptured),
+                    () => this.Finish(iCaptured));
+                this.subscriptions.Add(subscription);
+                ++i;
             }
         }
 
@@ -58,7 +63,19 @@ namespace Perspex.Styling
         {
             this.values[index] = value;
 
-            bool current = this.values.All(x => x);
+            bool current;
+            
+            switch (this.mode)
+            {
+                case ActivatorMode.And:
+                    current = this.values.All(x => x);
+                    break;
+                case ActivatorMode.Or:
+                    current = this.values.Any(x => x);
+                    break;
+                default:
+                    throw new InvalidOperationException("Invalid Activator mode.");
+            }
 
             if (current != last)
             {
@@ -69,10 +86,13 @@ namespace Perspex.Styling
 
         private void Finish(int i)
         {
-            if (!this.values[i])
+            // If the observable has finished on 'false' and we're in And mode then it will never 
+            // go back to true so we can unsubscribe from all the other subscriptions now. 
+            // Similarly in Or mode; if the completed value is true then we're done.
+            bool unsubscribe = this.mode == ActivatorMode.And ? !this.values[i] : this.values[i];
+
+            if (unsubscribe)
             {
-                // If the observable has finished on 'false' then it will never go back to true
-                // so we can unsubscribe from all the other subscriptions now.
                 foreach (IDisposable subscription in this.subscriptions)
                 {
                     subscription.Dispose();
diff --git a/Perspex/Styling/Selector.cs b/Perspex/Styling/Selector.cs
index 8d1e8d2c04..0ab8095d4a 100644
--- a/Perspex/Styling/Selector.cs
+++ b/Perspex/Styling/Selector.cs
@@ -53,7 +53,20 @@ namespace Perspex.Styling
 
         public Activator GetActivator(IStyleable control)
         {
-            return new Activator(this, control);
+            List<IObservable<bool>> inputs = new List<IObservable<bool>>();
+            Selector selector = this;
+            
+            while (selector != null)
+            {
+                if (selector.Observable != null)
+                {
+                    inputs.Add(selector.Observable(control));
+                }
+
+                selector = selector.Previous;
+            }
+
+            return new Activator(inputs);
         }
 
         public override string ToString()