diff --git a/Scripts/Editor/NodeEditorAction.cs b/Scripts/Editor/NodeEditorAction.cs
index dd49a06..315f23b 100644
--- a/Scripts/Editor/NodeEditorAction.cs
+++ b/Scripts/Editor/NodeEditorAction.cs
@@ -60,7 +60,7 @@ namespace XNodeEditor {
                 case EventType.MouseDrag:
                     if (e.button == 0) {
                         if (IsDraggingPort) {
-                            if (IsHoveringPort && hoveredPort.IsInput) {
+                            if (IsHoveringPort && hoveredPort.IsInput && draggedOutput.CanConnectTo(hoveredPort)) {
                                 if (!draggedOutput.IsConnectedTo(hoveredPort)) {
                                     draggedOutputTarget = hoveredPort;
                                 }
@@ -422,7 +422,7 @@ namespace XNodeEditor {
                 Rect fromRect;
                 if (!_portConnectionPoints.TryGetValue(draggedOutput, out fromRect)) return;
                 Vector2 from = fromRect.center;
-                col.a = 0.6f;
+                col.a = draggedOutputTarget != null ? 1.0f : 0.6f;
                 Vector2 to = Vector2.zero;
                 for (int i = 0; i < draggedOutputReroutes.Count; i++) {
                     to = draggedOutputReroutes[i];
diff --git a/Scripts/Editor/NodeEditorGUILayout.cs b/Scripts/Editor/NodeEditorGUILayout.cs
index 629c594..b136dc9 100644
--- a/Scripts/Editor/NodeEditorGUILayout.cs
+++ b/Scripts/Editor/NodeEditorGUILayout.cs
@@ -260,7 +260,7 @@ namespace XNodeEditor {
         /// <param name="serializedObject">The serializedObject of the node</param>
         /// <param name="connectionType">Connection type of added instance ports</param>
         /// <param name="onCreation">Called on the list on creation. Use this if you want to customize the created ReorderableList</param>
-        public static void InstancePortList(string fieldName, Type type, SerializedObject serializedObject, XNode.NodePort.IO io, XNode.Node.ConnectionType connectionType = XNode.Node.ConnectionType.Multiple, Action<ReorderableList> onCreation = null) {
+        public static void InstancePortList(string fieldName, Type type, SerializedObject serializedObject, XNode.NodePort.IO io, XNode.Node.ConnectionType connectionType = XNode.Node.ConnectionType.Multiple, XNode.Node.TypeConstraint typeConstraint = XNode.Node.TypeConstraint.None, Action<ReorderableList> onCreation = null) {
             XNode.Node node = serializedObject.targetObject as XNode.Node;
 
             Predicate<string> isMatchingInstancePort =
@@ -279,7 +279,7 @@ namespace XNodeEditor {
             // If a ReorderableList isn't cached for this array, do so.
             if (list == null) {
                 SerializedProperty arrayData = serializedObject.FindProperty(fieldName);
-                list = CreateReorderableList(fieldName, instancePorts, arrayData, type, serializedObject, io, connectionType, onCreation);
+                list = CreateReorderableList(fieldName, instancePorts, arrayData, type, serializedObject, io, connectionType, typeConstraint, onCreation);
                 if (reorderableListCache.TryGetValue(serializedObject.targetObject, out rlc)) rlc.Add(fieldName, list);
                 else reorderableListCache.Add(serializedObject.targetObject, new Dictionary<string, ReorderableList>() { { fieldName, list } });
             }
@@ -287,7 +287,7 @@ namespace XNodeEditor {
             list.DoLayoutList();
         }
 
-        private static ReorderableList CreateReorderableList(string fieldName, List<XNode.NodePort> instancePorts, SerializedProperty arrayData, Type type, SerializedObject serializedObject, XNode.NodePort.IO io, XNode.Node.ConnectionType connectionType, Action<ReorderableList> onCreation) {
+        private static ReorderableList CreateReorderableList(string fieldName, List<XNode.NodePort> instancePorts, SerializedProperty arrayData, Type type, SerializedObject serializedObject, XNode.NodePort.IO io, XNode.Node.ConnectionType connectionType, XNode.Node.TypeConstraint typeConstraint, Action<ReorderableList> onCreation) {
             bool hasArrayData = arrayData != null && arrayData.isArray;
             int arraySize = hasArrayData ? arrayData.arraySize : 0;
             XNode.Node node = serializedObject.targetObject as XNode.Node;
@@ -375,8 +375,8 @@ namespace XNodeEditor {
                     int i = 0;
                     while (node.HasPort(newName)) newName = fieldName + " " + (++i);
 
-                    if (io == XNode.NodePort.IO.Output) node.AddInstanceOutput(type, connectionType, newName);
-                    else node.AddInstanceInput(type, connectionType, newName);
+                    if (io == XNode.NodePort.IO.Output) node.AddInstanceOutput(type, connectionType, XNode.Node.TypeConstraint.None, newName);
+                    else node.AddInstanceInput(type, connectionType, typeConstraint, newName);
                     serializedObject.Update();
                     EditorUtility.SetDirty(node);
                     if (hasArrayData) arrayData.InsertArrayElementAtIndex(arraySize);
@@ -422,8 +422,8 @@ namespace XNodeEditor {
                     string newName = arrayData.name + " 0";
                     int i = 0;
                     while (node.HasPort(newName)) newName = arrayData.name + " " + (++i);
-                    if (io == XNode.NodePort.IO.Output) node.AddInstanceOutput(type, connectionType, newName);
-                    else node.AddInstanceInput(type, connectionType, newName);
+                    if (io == XNode.NodePort.IO.Output) node.AddInstanceOutput(type, connectionType, typeConstraint, newName);
+                    else node.AddInstanceInput(type, connectionType, typeConstraint, newName);
                     EditorUtility.SetDirty(node);
                     instancePortCount++;
                 }
diff --git a/Scripts/Node.cs b/Scripts/Node.cs
index 43d716e..7c06f92 100644
--- a/Scripts/Node.cs
+++ b/Scripts/Node.cs
@@ -41,6 +41,16 @@ namespace XNode {
             Override,
         }
 
+        /// <summary> Tells which types of input to allow </summary>
+        public enum TypeConstraint {
+            /// <summary> Allow all types of input</summary>
+            None,
+            /// <summary> Allow similar and inherited types </summary>
+            Inherited,
+            /// <summary> Allow only similar types </summary>
+            Strict,
+        }
+
         /// <summary> Iterate over all ports on this node. </summary>
         public IEnumerable<NodePort> Ports { get { foreach (NodePort port in ports.Values) yield return port; } }
         /// <summary> Iterate over all outputs on this node. </summary>
@@ -87,21 +97,21 @@ namespace XNode {
         /// <summary> Convenience function. </summary>
         /// <seealso cref="AddInstancePort"/>
         /// <seealso cref="AddInstanceOutput"/>
-        public NodePort AddInstanceInput(Type type, Node.ConnectionType connectionType = Node.ConnectionType.Multiple, string fieldName = null) {
-            return AddInstancePort(type, NodePort.IO.Input, connectionType, fieldName);
+        public NodePort AddInstanceInput(Type type, Node.ConnectionType connectionType = Node.ConnectionType.Multiple, Node.TypeConstraint typeConstraint = TypeConstraint.None, string fieldName = null) {
+            return AddInstancePort(type, NodePort.IO.Input, connectionType, typeConstraint, fieldName);
         }
 
         /// <summary> Convenience function. </summary>
         /// <seealso cref="AddInstancePort"/>
         /// <seealso cref="AddInstanceInput"/>
-        public NodePort AddInstanceOutput(Type type, Node.ConnectionType connectionType = Node.ConnectionType.Multiple, string fieldName = null) {
-            return AddInstancePort(type, NodePort.IO.Output, connectionType, fieldName);
+        public NodePort AddInstanceOutput(Type type, Node.ConnectionType connectionType = Node.ConnectionType.Multiple, Node.TypeConstraint typeConstraint = TypeConstraint.None, string fieldName = null) {
+            return AddInstancePort(type, NodePort.IO.Output, connectionType, typeConstraint, fieldName);
         }
 
         /// <summary> Add a dynamic, serialized port to this node. </summary>
         /// <seealso cref="AddInstanceInput"/>
         /// <seealso cref="AddInstanceOutput"/>
-        private NodePort AddInstancePort(Type type, NodePort.IO direction, Node.ConnectionType connectionType = Node.ConnectionType.Multiple, string fieldName = null) {
+        private NodePort AddInstancePort(Type type, NodePort.IO direction, Node.ConnectionType connectionType = Node.ConnectionType.Multiple, Node.TypeConstraint typeConstraint = TypeConstraint.None, string fieldName = null) {
             if (fieldName == null) {
                 fieldName = "instanceInput_0";
                 int i = 0;
@@ -110,7 +120,7 @@ namespace XNode {
                 Debug.LogWarning("Port '" + fieldName + "' already exists in " + name, this);
                 return ports[fieldName];
             }
-            NodePort port = new NodePort(fieldName, type, direction, connectionType, this);
+            NodePort port = new NodePort(fieldName, type, direction, connectionType, typeConstraint, this);
             ports.Add(fieldName, port);
             return port;
         }
@@ -216,14 +226,18 @@ namespace XNode {
             public ShowBackingValue backingValue;
             public ConnectionType connectionType;
             public bool instancePortList;
+            public TypeConstraint typeConstraint;
 
             /// <summary> Mark a serializable field as an input port. You can access this through <see cref="GetInputPort(string)"/> </summary>
             /// <param name="backingValue">Should we display the backing value for this port as an editor field? </param>
             /// <param name="connectionType">Should we allow multiple connections? </param>
-            public InputAttribute(ShowBackingValue backingValue = ShowBackingValue.Unconnected, ConnectionType connectionType = ConnectionType.Multiple, bool instancePortList = false) {
+            /// <param name="typeConstraint">Constrains which input connections can be made to this port </param>
+            /// <param name="instancePortList">If true, will display a reorderable list of inputs instead of a single port. Will automatically add and display values for lists and arrays </param>
+            public InputAttribute(ShowBackingValue backingValue = ShowBackingValue.Unconnected, ConnectionType connectionType = ConnectionType.Multiple, TypeConstraint typeConstraint = TypeConstraint.None, bool instancePortList = false) {
                 this.backingValue = backingValue;
                 this.connectionType = connectionType;
                 this.instancePortList = instancePortList;
+                this.typeConstraint = typeConstraint;
             }
         }
 
@@ -237,6 +251,7 @@ namespace XNode {
             /// <summary> Mark a serializable field as an output port. You can access this through <see cref="GetOutputPort(string)"/> </summary>
             /// <param name="backingValue">Should we display the backing value for this port as an editor field? </param>
             /// <param name="connectionType">Should we allow multiple connections? </param>
+            /// <param name="instancePortList">If true, will display a reorderable list of outputs instead of a single port. Will automatically add and display values for lists and arrays </param>
             public OutputAttribute(ShowBackingValue backingValue = ShowBackingValue.Never, ConnectionType connectionType = ConnectionType.Multiple, bool instancePortList = false) {
                 this.backingValue = backingValue;
                 this.connectionType = connectionType;
diff --git a/Scripts/NodeDataCache.cs b/Scripts/NodeDataCache.cs
index 8d96146..434ffc5 100644
--- a/Scripts/NodeDataCache.cs
+++ b/Scripts/NodeDataCache.cs
@@ -30,7 +30,7 @@ namespace XNode {
                 NodePort staticPort;
                 if (staticPorts.TryGetValue(port.fieldName, out staticPort)) {
                     // If port exists but with wrong settings, remove it. Re-add it later.
-                    if (port.connectionType != staticPort.connectionType || port.IsDynamic || port.direction != staticPort.direction) ports.Remove(port.fieldName);
+                    if (port.connectionType != staticPort.connectionType || port.IsDynamic || port.direction != staticPort.direction || port.typeConstraint != staticPort.typeConstraint) ports.Remove(port.fieldName);
                     else port.ValueType = staticPort.ValueType;
                 }
                 // If port doesn't exist anymore, remove it
diff --git a/Scripts/NodePort.cs b/Scripts/NodePort.cs
index 3f55795..24e4941 100644
--- a/Scripts/NodePort.cs
+++ b/Scripts/NodePort.cs
@@ -21,6 +21,7 @@ namespace XNode {
 
         public IO direction { get { return _direction; } }
         public Node.ConnectionType connectionType { get { return _connectionType; } }
+        public Node.TypeConstraint typeConstraint { get { return _typeConstraint; } }
 
         /// <summary> Is this port connected to anytihng? </summary>
         public bool IsConnected { get { return connections.Count != 0; } }
@@ -49,6 +50,7 @@ namespace XNode {
         [SerializeField] private List<PortConnection> connections = new List<PortConnection>();
         [SerializeField] private IO _direction;
         [SerializeField] private Node.ConnectionType _connectionType;
+        [SerializeField] private Node.TypeConstraint _typeConstraint;
         [SerializeField] private bool _dynamic;
 
         /// <summary> Construct a static targetless nodeport. Used as a template. </summary>
@@ -61,6 +63,7 @@ namespace XNode {
                 if (attribs[i] is Node.InputAttribute) {
                     _direction = IO.Input;
                     _connectionType = (attribs[i] as Node.InputAttribute).connectionType;
+                    _typeConstraint = (attribs[i] as Node.InputAttribute).typeConstraint;
                 } else if (attribs[i] is Node.OutputAttribute) {
                     _direction = IO.Output;
                     _connectionType = (attribs[i] as Node.OutputAttribute).connectionType;
@@ -75,17 +78,19 @@ namespace XNode {
             _direction = nodePort.direction;
             _dynamic = nodePort._dynamic;
             _connectionType = nodePort._connectionType;
+            _typeConstraint = nodePort._typeConstraint;
             _node = node;
         }
 
         /// <summary> Construct a dynamic port. Dynamic ports are not forgotten on reimport, and is ideal for runtime-created ports. </summary>
-        public NodePort(string fieldName, Type type, IO direction, Node.ConnectionType connectionType, Node node) {
+        public NodePort(string fieldName, Type type, IO direction, Node.ConnectionType connectionType, Node.TypeConstraint typeConstraint, Node node) {
             _fieldName = fieldName;
             this.ValueType = type;
             _direction = direction;
             _node = node;
             _dynamic = true;
             _connectionType = connectionType;
+            _typeConstraint = typeConstraint;
         }
 
         /// <summary> Checks all connections for invalid references, and removes them. </summary>
@@ -240,6 +245,23 @@ namespace XNode {
             return false;
         }
 
+        /// <summary> Returns true if this port can connect to specified port </summary>
+        public bool CanConnectTo(NodePort port) {
+            // Figure out which is input and which is output
+            NodePort input = null, output = null;
+            if (IsInput) input = this;
+            else output = this;
+            if (port.IsInput) input = port;
+            else output = port;
+            // If there isn't one of each, they can't connect
+            if (input == null || output == null) return false;
+            // Check type constraints
+            if (input.typeConstraint == XNode.Node.TypeConstraint.Inherited && !input.ValueType.IsAssignableFrom(output.ValueType)) return false;
+            if (input.typeConstraint == XNode.Node.TypeConstraint.Strict && input.ValueType != output.ValueType) return false;
+            // Success
+            return true;
+        }
+
         /// <summary> Disconnect this port from another port </summary>
         public void Disconnect(NodePort port) {
             // Remove this ports connection to the other