This chapter will show you how to use and specialise UML diagrams for describing the user interfaces of a software system. In order to accomplish the description of user interfaces, the proposed technique considers three specialised UML diagrams called user-interaction, user-interface, and GUI-class diagrams, which will be built following a model-driven development (MDD) perspective. These diagrams can be seen as the UML-based UI models of the system. In addition, this chapter is concerned with code-generation to implement the user interfaces of the system by using GUI-class diagrams and user-interaction diagrams. A case study of an Internet book shopping system is introduced in this chapter to proof and illustrate the proposed user interaction and interface design technique.