Controls in engineering are efforts to change, design, or modify the behaviors of dynamic systems. Automatic control is control that involves only machines and devices, and that has no human intervention. Examples of automatic control are diverse, including room-temperature control, cruise control of cars, missile guidance, trajectory control of robots, control of appliances such as washing machines and refrigerators, and control of industrial processes like papermaking and steelmaking. In this chapter, for simplicity, an automatic control system is called a control system. Because the focus of this text is on the modeling and analysis of dynamic systems, only basic concepts of feedback control are introduced. For theories and methods about feedback control systems, one may refer to standard textbooks for control courses, including those listed at the end of the chapter.