Consideration is given to the problem of controlling the output speed of a hydraulic transmission system, when the main source of disturbance is the changing speed of the prime-mover. A control system is described which will maintain a constant output speed over a range of input speeds of the order of 10:1. It is shown that satisfactory performance cannot be achieved over this wide speed range by using normal feedback control. The addition of a feedforward control operation overcomes this difficulty and yields a system which offers high performance. This system is particularly advantageous when the moment of inertia of the driven load is high. The control system has been realised in the form of an electro-hydraulic system and the results of tests on a typical transmission system are presented.