MARIN is designing and building a modular autonomous underwater vehicle (mAUV). The vehicle will serve as a platform for research projects on autonomous vehicles, to be carried out in our model 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 discusses the design and construction of the vehicle. The overall design of the vehicle is discussed, as well as the selected actuators and all internal mechanical, electrical and instrumentation systems.