Abstract: A water leak detection and prevention system uses sensors, wires, and wireless transmitters, to monitor all plumbing fixtures (faucets, toilets, hose bibs, etc.) and water-using appliances (dishwashers, washing machines, icemakers, etc.). When water is being used at a plumbing fixture or a water-using appliance, the sensor shows water that is expected to be used at that location and communicates the usage and the amount of flow for that location. Excessive usage or flow, or any flow when a location has not called for water, is indicative of a leak. Water to the building is on all the time—water pressure remains in the plumbing—creating no delay in water delivery. Water pressure and water flow are monitored at all times whether water is off or on. Should a leak occur the water flow sensor will show that water is flowing and that no approved location has called for water. The main valve will immediately be turned off and water in the plumbing system will be purged safely outside the building.