Abstract: An anti-siphon and back-flow prevention valve having a main valve chamber including air ports, an inlet at its lower end and an outlet at its upper end. Contained within the main valve chamber is a check valve including a check valve seat, through which the flow of liquid in the valve must pass. The check valve seat has a flexible diaphragm which mates with the check valve seat and functions to block any back flow of liquid from the main valve chamber into the inlet. A flexible ring is also contained in the main valve chamber and mates with the air ports to function as an air valve, allowing air to enter the main valve chamber during low pressure conditions down stream from the valve. The check valve and air valve operate independently of each other, responding to the various air and liquid pressures in and around the main valve chamber. Both valves are normally held closed by the force of gravity and static water pressure within the main valve chamber during no-flow conditions.