A system for transmitting power to the wheels of a ground vehicle must be able to meet all the possible torque and speed requirements likely to be encountered in normal use The hydrostatic system offers a stepless transmission of power with a high response rate, and can cater for a wide range of torque and speed combinations. However, because of its low efficiency, rarely greater than 80%, the use of the hydrostatic system tends to be restricted to specialist applications. The paper describes the various loss models for the axial piston pump of the swashplate type which is used widely in modern hydrostatic systems. These models have been incorporated in a performance prediction method that could help to minimise the losses at the design stage. The predicted and measured performance data from a pump of modern design have been compared. The method may be applied also, with small modifications, to axial piston motors of similar design.