A major challenge for software engineers in the coming years is to develop systems which users can learn quickly and operate effectively. The user-computer interface is a major factor in determining whether a software system can be readily mastered by the general user population. Rapid prototyping is a software methodology that allows the interface designer to iteratively design and evaluate the learnability and effectiveness of several user interfaces with the intended users. This iterative approach to evaluation and modification of the user interface can be done early in the system development cycle, and therefore, it can be accomplished without costly software modifications to the application. We present here a tool, the Graphical User Interface DEsign System (GUIDES), for rapid prototyping of dynamic user-computer interfaces, and describe how it can be used to increase software productivity.