Copyright © 2003 IFAC. Tethra is a prototype AUV, designed at the University of Limerick to carry out research in underwater robotics and control. The vehicle adopts a behaviour based control paradigm. In this paper we report on the design and development of the Tethra test platform, it's low-level control functionality and how this low-level control functionality is incorporated within the robot controller. The functioning test platform is being used in ongoing research into controller development, investigating classical and neural network approaches. The vehicle is also being used for further behaviour based control development.