A non-return valve, swing check valve, or one-way valve is a mechanical device, which normally allows fluid (liquid or gas) to flow through it in only one direction and in which the disc, the movable part to block the flow, swings on a hinge either onto the seat to block reverse flow or off the seat to allow forward flow. The seat opening cross-section may be perpendicular to the centerline between the two ports or at an angle.

Check valves/Non Return Valves are two-port valves, meaning they have two openings in the body, one for fluid to enter and the other for fluid to leave. There are various types of check valves used in a wide variety of applications.

An important concept in check valves is the cracking pressure which is the minimum upstream pressure at which the valve will operate. Typically the check valve is designed for and can therefore be specified for a specific cracking pressure.