Current interactive user interface construction tools are often more of a hindrance than a benefit during the early stages of user interface design. These tools take too much time to use and force designers to specify more of the design details than they wish at this early stage. Most interface designers, especially those who have a background in graphic design, prefer to sketch early interface ideas on paper or on a whiteboard. We are developing an interactive tool called SILK that allows designers to quickly sketch an interface using an electronic pad and stylus. SILK preserves the important properties of pencil and paper: a rough drawing can be produced very quickly and the medium is very flexible. However, unlike a paper sketch, this electronic sketch is interactive and can easily be modified. In addition, our system allows designers to examine, annotate, and edit a complete history of the design. When the designer is satisfied with this early prototype, SILK can transform the sketch into a complete, operational interface in a specified look-and-feel. This transformation is guided by the designer. By supporting the early phases of the interface design life cycle, our tool should both ease the development of user interface prototypes and reduce the time needed to create a final interface. This paper describes our prototype and provides design ideas for a production-level system.