======================================== .NET Framework 3.0 Custom Control ======================================== Follow steps 1a or 1b and then 2 to use this custom control in a XAML file. Step 1a) Using this custom control in a XAML file that exists in the current project. Add this XmlNamespace attribute to the root element of the markup file where it is to be used: xmlns:MyNamespace="clr-namespace:CloseableTabItemDemo" Step 1b) Using this custom control in a XAML file that exists in a different project. Add this XmlNamespace attribute to the root element of the markup file where it is to be used: xmlns:MyNamespace="clr-namespace:CloseableTabItemDemo;assembly=CloseableTabItemDemo" You will also need to add a project reference from the project where the XAML file lives to this project and Rebuild to avoid compilation errors: Right click on the target project in the Solution Explorer and "Add Reference"->"Projects"->[Browse to and select this project] Step 2) Go ahead and use your control in the XAML file. Note that Intellisense in the XML editor does not currently work on custom controls and its child elements.