diff --git a/Source/Collections/MultiDictionary.Test.cs b/Source/Collections/MultiDictionary.Test.cs
index 19310bc..f950252 100644
--- a/Source/Collections/MultiDictionary.Test.cs
+++ b/Source/Collections/MultiDictionary.Test.cs
@@ -41,6 +41,20 @@ namespace Nuclex.Support.Collections {
       Assert.IsNotNull(dictionary); // nonsense, prevents compiler warning
     }
 
+    /// <summary>
+    ///   Verifies that the count is initialized correctly when building
+    ///   a multi dictionary from a dictionary of value collections.
+    /// </summary>
+    [Test]
+    public void CountIsCalculatedIfInitializedFromDictionary() {
+      var contents = new Dictionary<int, ICollection<string>>();
+      contents.Add(1, new List<string>(new string[] { "one", "eins" }));
+      contents.Add(2, new List<string>(new string[] { "two", "zwei" }));
+
+      var multiDictionary = new MultiDictionary<int, string>(contents);
+      Assert.AreEqual(4, multiDictionary.Count);
+    }
+
     /// <summary>
     ///   Verifies that a new multi dictionary based on a read-only dictionary is
     ///   also read-only
@@ -116,6 +130,15 @@ namespace Nuclex.Support.Collections {
       Assert.AreEqual(1, dictionary.CountValues(30));
     }
 
+    /// <summary>
+    ///   Verifies that counting the values of a non-existing key returns 0
+    /// </summary>
+    [Test]
+    public void CountingValuesOfNonExistentKeyReturnsNull() {
+      var dictionary = new MultiDictionary<int, string>();
+      Assert.AreEqual(0, dictionary.CountValues(1));
+    }
+
     /// <summary>
     ///   Ensures that its possible to remove values individually without affecting
     ///   other values stored under the same key
@@ -150,10 +173,39 @@ namespace Nuclex.Support.Collections {
       dictionary.Add(10, "zehn");
 
       Assert.AreEqual(2, dictionary.Count);
-      var collectionOfCollections = (ICollection<KeyValuePair<int, ICollection<string>>>)dictionary;
+      var collectionOfCollections =
+        (ICollection<KeyValuePair<int, ICollection<string>>>)dictionary;
       Assert.AreEqual(1, collectionOfCollections.Count);
     }
 
+    /// <summary>
+    ///   Verifies that the multi dictionary can be tested for containment of a specific value
+    /// </summary>
+    [Test]
+    public void ContainmentCanBeTested() {
+      var dictionary = new MultiDictionary<int, string>();
+      dictionary.Add(10, "ten");
+      dictionary.Add(10, "zehn");
+
+      Assert.IsTrue(dictionary.Contains(new KeyValuePair<int, string>(10, "ten")));
+      Assert.IsTrue(dictionary.Contains(new KeyValuePair<int, string>(10, "zehn")));
+      Assert.IsFalse(dictionary.Contains(new KeyValuePair<int, string>(10, "dix")));
+      Assert.IsFalse(dictionary.Contains(new KeyValuePair<int, string>(20, "ten")));
+    }
+
+    /// <summary>
+    ///   Verifies that the multi dictionary can be tested for containment of a specific key
+    /// </summary>
+    [Test]
+    public void KeyContainmentCanBeTested() {
+      var dictionary = new MultiDictionary<int, string>();
+      dictionary.Add(10, "ten");
+      dictionary.Add(10, "zehn");
+
+      Assert.IsTrue(dictionary.ContainsKey(10));
+      Assert.IsFalse(dictionary.ContainsKey(20));
+    }
+
   }
 
 } // namespace Nuclex.Support.Collections
diff --git a/Source/Collections/MultiDictionary.cs b/Source/Collections/MultiDictionary.cs
index 5d335b3..9674990 100644
--- a/Source/Collections/MultiDictionary.cs
+++ b/Source/Collections/MultiDictionary.cs
@@ -333,15 +333,39 @@ namespace Nuclex.Support.Collections {
 
         ICollection<TValue> currentValues;
         if(this.typedDictionary.TryGetValue(key, out currentValues)) {
-          currentValues.Clear();
+          ValueList currentValueList = (ValueList)currentValues;
+
+          int index = 0;
+          foreach(TValue addedValue in value) {
+            if(index < currentValueList.Count) {
+              TValue original = currentValueList[index];
+              currentValueList[index] = addedValue;
+              OnReplaced(
+                new KeyValuePair<TKey, TValue>(key, original),
+                new KeyValuePair<TKey, TValue>(key, addedValue)
+              );
+            } else {
+              currentValueList.Add(addedValue);
+              OnAdded(new KeyValuePair<TKey, TValue>(key, addedValue));
+            }
+            ++index;
+          }
+
+          int count = currentValueList.Count;
+          while(count > index) {
+            --count;
+            TValue removedValue = currentValueList[count];
+            currentValueList.RemoveAt(count);
+            OnRemoved(new KeyValuePair<TKey, TValue>(key, removedValue));
+          }
         } else {
           currentValues = new ValueList(this);
           this.typedDictionary.Add(key, currentValues);
-        }
 
-        foreach(TValue addedValue in value) {
-          currentValues.Add(addedValue);
-          OnAdded(new KeyValuePair<TKey, TValue>(key, addedValue));
+          foreach(TValue addedValue in value) {
+            currentValues.Add(addedValue);
+            OnAdded(new KeyValuePair<TKey, TValue>(key, addedValue));
+          }
         }
       }
     }
@@ -420,6 +444,13 @@ namespace Nuclex.Support.Collections {
     /// <param name="item">Item that has been removed from the collection</param>
     protected virtual void OnRemoved(KeyValuePair<TKey, TValue> item) { }
 
+    /// <summary>Fires the 'ItemReplaced' event</summary>
+    /// <param name="oldItem">Item that was replaced in the collection</param>
+    /// <param name="newItem">Item that the original was replaced by</param>
+    protected virtual void OnReplaced(
+      KeyValuePair<TKey, TValue> oldItem, KeyValuePair<TKey, TValue> newItem
+    ) { }
+
     /// <summary>Fires the 'Clearing' event</summary>
     protected virtual void OnClearing() { }
 
diff --git a/Source/Collections/ObservableSet.Test.cs b/Source/Collections/ObservableSet.Test.cs
index ddcaf36..a2954d1 100644
--- a/Source/Collections/ObservableSet.Test.cs
+++ b/Source/Collections/ObservableSet.Test.cs
@@ -31,12 +31,40 @@ using NMock;
 
 namespace Nuclex.Support.Collections {
 
-#if false
   /// <summary>Unit Test for the observable set wrapper</summary>
   [TestFixture]
   internal class ObservableSetTest {
+
+    #region interface IObservableCollectionSubscriber<TItem>
+
+    public interface IObservableCollectionSubscriber<TItem> {
+
+      /// <summary>Raised when an item has been added to the collection</summary>
+      event EventHandler<ItemEventArgs<TItem>> ItemAdded;
+      /// <summary>Raised when an item is removed from the collection</summary>
+      event EventHandler<ItemEventArgs<TItem>> ItemRemoved;
+      /// <summary>Raised when an item is replaced in the collection</summary>
+      event EventHandler<ItemReplaceEventArgs<TItem>> ItemReplaced;
+      /// <summary>Raised when the collection is about to be cleared</summary>
+      event EventHandler Clearing;
+      /// <summary>Raised when the collection has been cleared</summary>
+      event EventHandler Cleared;
+
+    }
+
+    #endregion // interface IObservableCollectionSubscriber<TItem>
+
+    
+
+    /// <summary>
+    ///   Verifies that the observable set has a default constructor
+    /// </summary>
+    [Test]
+    public void HasDefaultConstructor() {
+      Assert.IsNotNull(new ObservableSet<int>());
+    }
+
   }
-#endif
 
 } // namespace Nuclex.Support.Collections
 
diff --git a/Source/Collections/ObservableSet.cs b/Source/Collections/ObservableSet.cs
index f142888..76ad175 100644
--- a/Source/Collections/ObservableSet.cs
+++ b/Source/Collections/ObservableSet.cs
@@ -82,7 +82,11 @@ namespace Nuclex.Support.Collections {
     ///   True if the element was added, false if it was already contained in the set
     /// </returns>
     public bool Add(TItem item) {
-      return this.set.Add(item);
+      bool wasAdded = this.set.Add(item);
+      if(wasAdded) {
+        OnAdded(item);
+      }
+      return wasAdded;
     }
 
     /// <summary>Removes all elements that are contained in the collection</summary>