In control systems, the output of the controller (also of the equalizer or compensator), which operates on the command (and feedback) input signals, is invariably sent to an actuator. œe latter is a power element critically positioned at the plant input and, to a large extent, determines the performance of the overall control system. œus, the importance of the accurate modeling of the actuator element with respect to achieving and maintaining the desired control system performance over its operating range cannot be overemphasized. œe impact of items such as the actuator phase-lag characteristics, parameter variations and aging, sensor noise, and operating scenario (e.g., in a žight control context, it is referred to as ¦ight conditions) should be explicitly considered in a control system design process. To achieve a robust control system, where these items are taken into account, requires a good understanding and an accurate model of the actuation element. œis chapter discusses the actuator modeling process.