Windows Phone 7 Series UI Design & Interaction Guide
“A clear, straightforward design not only makes an application legible, it encourages usage. This guide will provide design knowledge and fundamentals for this type of UI development. We highly recommend that developers adopt the Metro design style whenever possible. Although requirements may vary based on the application, paralleling this experience will create a more consistent, fluid UI experience from the custom and built-in application view.” (The Windows Phone Developers Blog)