Valve selection is based on function, material suitability, design pressure/temperature extremities, plant life, end connections, operation, weight, availability, maintenance, and cost. If the valve does not meet the design criteria, then even if it is free, it is still too expensive, because of the costs to replace it when it fails. Just like life, valve selection is a series of compromises.

Valves are the components in a fluid flow or pressure system that regulate either the flow or the pressure of the fluid. This duty may involve stopping and starting flow, controlling the flow rate, diverting flow, preventing backflow, controlling pressure, or relieving pressure.

These duties are performed by adjusting the position of the closure member in the valve. This may be done either manually or automatically. The manual operation also includes the operation of the valve by means of a manually controlled power operator. Manually operated valves are usually for stopping and starting flow, controlling the flow rate, and diverting flow; and automatically operated valves are usually for preventing backflow and relieving pressure. The manually operated valves are referred to as manual valves, while valves for the prevention of backflow and the relief of pressure are referred to as check valves and pressure relief valves, respectively.