MARIN is designing and building a modular autonomous underwater vehicle (mAUV). The vehicle will serve as a platform for research projects to be carried out in our model test basins. The modular design will enable future extensions with additional sensors and actuators. The design of the vehicle and its control system is aimed at combining capabilities for position keeping (zero speed) and track sailing (medium and high speed). The vehicle is controlled in all 6 degrees of freedom, without any a-priori limitations. This paper describes the control design of the mAUV. A feedback controller is designed and its robust stability analysed under parametric uncertainty and dynamic uncertainty due to the non-linear behaviour of its thrusters and the manoeuvring characteristics. As the system is not limited to small angles, the attitude is determined with quaternions to avoid singularities. Simulations show that the controlled system performs well at keeping its track, but currently the tracking performance degrades at at higher velocities.