Hydraulic servos are characterised by their high-performance nature but due to their size and weight are not suitable for robotics where new legged applications require high power density and excellent dynamic behaviour in a small size. As an answer to this need a new class of integrated smart actuators is being developed. These systems consist of a servo valve, hydraulic cylinder, sensors and a controller all in a single device. This paper outlines the detailed modelling of the smart actuator for use in simulation and control design. The result is a model consisting of the dynamics of the novel ultra-low leakage servovalve, the valve flow characteristics considering the properties of each spool land, the single-ended cylinder with friction and the pressure losses in the supply and return lines to the actuator. The models are a combination of empirical and theoretical development, validated with experimental data. The smart actuator’s unique properties; compactness, weight and efficiency, combined with high-performance hydraulics make it well suited to mobile robot applications.