Electrohydraulic fluid power systems, combined with electronic controls, provide fluid power systems designers with a greater level of machine accuracy, efficiency, versatility, and reliability than ever before available. Servo-valve technology is now widely accepted in industry, and users and manufacturers have gained considerable experience adapting from aerospace to industrial applications. Broadly speaking, three types of electronic control systems are presently available: open loop, simple closed loop, and sophisticated closed loop. Servo-controlled hydraulic components include several types of directional valves, cylinders, fluid motors, and hydrostatic drive-stroke controllers. Electrohydraulic stepping motors also use open-loop control, but their use is limited. The chapter reviews the design and operation of the various components that make up a typical electrohydraulic servo system. It includes hydraulic amplifiers, servo valves, proportional valves, and feedback elements. The purpose of a feedback control system is to monitor an output in a manner dictated by an input in the presence of spurious disturbances.