Generally the pressure level in hydraulic systems will vary so as to provide the required torque or force from an actuator in order to drive an external load from the particular application. During start and stopping situations and when the load is varying transiently the pressure may exceed the maximum safe value for the system. In many systems several actuators will be driven by a single pump and, in addition to limiting the supply pressure it may be necessary to reduce the pressure level supplied to individual services. There are many different types of valves on the market and this chapter describes some of these and the operating principles involved.