In this paper we describe a physical user interface system for easy and natural user-computer interaction. VisualPen is a vision-based system for real-time detection and tracking of a stylus that completely replaces mouse and keyboard, thus providing a valid input device for mobile computers, and its low computational complexity renders it suitable also for PDAs. The system can be operated from a wide range of distances (either from a desk or from a wall-mounted projection panel) and is able to work with all lightning conditions. The architecture of the system is here described, and experimental results in several tests are presented and commented.