|Abstract||The reacTIVision system is software for tracking specially designed fiducials (markers) in a real-time video stream. ReacTIVision was designed to enable expressive gestural control of musical sound, and can track many markers at a high frame rate. The development of reacTIVision involved not only computer vision algorithms, but also the design of a new marker system. We co-designed the computer vision system and markers, applying evolutionary computation to minimise marker size while meeting geometric constraints required to efficiently compute the location and 2D orientation of the markers. The computer vision techniques adopted would not have been practical had the genetic algorithms not been able to solve these constraints. This paper focuses on the design and evolution of the markers, the computer vision aspects of reacTIVision are documented elsewhere.