The Tower is a modular development system for designing and prototyping computational devices. Physically, the Tower consists of a primary foundation layer with a central processor. At this point, two foundations are available, equipped with either the PIC16F877 microcontroller from Microchip Inc.�, or the Rabbit� 2300 processor from Rabbit Semiconductor�. Additional modules can be added to the stack as needed by specific applications, and a special prototyping layer allows for simplified design of new modules for the system. Currently, layers are being produced that allow for sensor readings, servo-motor control, MIDI music playback, data collection, network connectivity, and RF and infrared communication. In addition to the currently available layers, we are actively designing a set of hardware and software development tools to make it easy for anyone to add their own layers to the system as an application's needs require.