Abstract: A pool security system incorporates a passive infrared element and an underwater acoustic element. The passive infrared detection element generates a thin infrared layer which overlays the entire water surface area of the pool. As a heat generating body passes through the infrared layer, the infrared element detects the body and generates a first detect signal. As the body enters the water, it causes waves which propagate through the water. These waves are detected by the acoustic element. The acoustic element continues to receive waves generated as the body struggles at or below the water surface. A master control circuit is coupled to the infrared and the acoustic elements to receive the first and second detect signals. The master control circuit is designed to detect when the first detect signal is received, followed a predetermined time by the second detect signal. When this occurs, the master control circuit will generate an alarm signal, thus alerting others of the danger of unauthorized entry.