Publisher Summary Hydraulic or pneumatic system is concerned with moving, gripping, or applying force to an object. Devices that actually achieve this objective are called actuators, and can be split into three basic types. This chapter discusses linear and rotary actuators. Linear actuators are used to move an object or apply a force in a straight line. Rotary actuators are the hydraulic and pneumatic equivalent of an electric motor. The third type of actuator is used to operate flow control valves for process control of gases, liquids or steam. These actuators are generally pneumatically operated and are described with process control pneumatics. The operational speed of an actuator is determined by the fluid flow rate and the actuator area (for a cylinder) or the displacement (for a motor).