One of the fundamental design goals of Silverlight and WPF is to enable developers to be able to easily encapsulate UI functionality into re-usable controls. You can implement new custom controls by deriving a class from one of the existing Control classes (either a Control base class or from a control like TextBox, Button, etc). Alternatively you can create re-usable User Controls - which make it easy to use a XAML markup file to compose a control's UI (and which makes them super easy to build). In Part 6 of my Digg.com tutorial blog series I showed how to create a new user control using VS 2008's "Add New Item" project item dialog and by then defining UI within it. This approach works great when you know up front that you want to encapsulate UI in a user control. You can also use the same technique with Expression Blend.