Valves are an essential part of any piping system that conveys liquids, gases, vapors, slurries and mixtures of liquid, and gaseous phases of various flow media. Starting with primitive means for stopping, allowing, or diverting water-flow from a source through shallow or deep channels, such as wooden or stone wedges stuck between the edges of a water passage, man has developed several different types of simple and sophisticated valve designs. Different types of valves include: gate, globe, check, ball, plug, butterfly, diaphragm, pinch, pressure relief, and control valves. Each of these types has several categories and designs, each offering different features and functional capabilities. Some valves are self-actuated while others are manually operated or have actuators that are powered with electric motors, are pneumatic or hydraulic, or a combination to operate the valve. Valves are manufactured with metals and nonmetals. Valves are used in industrial piping systems, transportation and distribution pipelines, building services piping, civic facilities, and irrigation piping systems. The piping systems of industrial, commercial, residential, and other civic facilities carry the lifeblood of modern civilization, like arteries and veins. And the valves in those piping systems serve the functions of allowing, stopping, regulating, and controlling the flow, to fulfill the intended objectives of the system. When fluid pressure builds up beyond a set limit, the valves relieve the over pressure to safeguard the integrity of the piping system or a component. Valves help in maintaining or breaking a vacuum. Valves also assist in maintaining the pressure or temperature of the flow medium within the desired range or limit. In order to select a suitable valve for a particular application, the user must evaluate the valve characteristics, including the design features, materials of construction, and performance, in light of specific application requirements: flow medium, process design requirements, piping design criteria, and economic factors. This chapter provides a brief discussion of topics considered in the selection and application of commonly used valves. #Little_PEng