Abstract: A steering system for a vehicle having tracks or sets of wheels on opposite sides thereof, comprising a differential drive for the tracks or wheel sets which includes a hydrostatic transmission, and a control which is operative in response to a steering demand to cause the hydraulic transmission to drive the tracks or wheel sets at different relative speeds and thereby steer the vehicle. When the pressure in the hydrostatic transmission rises above a predetermined level during such steering, a brake is automatically actuated to apply a braking effect to the appropriate track or wheel set, thereby supplementing the steering action. The action of the brake is progressive, i.e. it applies an increasing braking effect with increased steering demand.