Hydraulic systems use either linear or rotary actuators to deploy potential energy, carried by fluid pressure and flow rate, by converting it to mechanical forms of power in force and velocity for performing designated work. This chapter gives detailed explanations of the operating and control principles of different types of hydraulic cylinders and motors. This chapter also introduces the basic principle of hydrostatic transmission, a special type of hydraulic power deployment widely applied on many off-road vehicles, by coupling pump and motor(s) in a system to transmit power from the prime mover directly to drive wheels.