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Reliably detecting people hidden in an enclosed space such as a motor vehicle, container, railroad car, etc. is an important aspect of a variety of security procedures. in this paper we describe a system called an Enclosed Space Detector (ESD) which operates by detecting a person's ballistocardiogram. Conceived initially for use by the U.S. Border Patrol to aid in detecting people smuggled into the U.S. in motor vehicles it also has applications in prisons, defense plants, nuclear plants or any other facility with high security requirements. The ESD's principle of operation is based upon the fact that with each beat of the human heart a force is imparted to the reminder of the body. This force, called the ballistocardiogram, is in turn coupled to any surface with which the body is in contact. The motion of the surface in response to the force although small is detectable with sophisticated signal processing procedures. The ESD employs a sensitive geophone. The resulting signal is first digitized and then filtered. The power in several overlapping time increments is then calculated. The detection criteria is based upon the observation that if the enclosure is enpty the signal will be primarily random noise and the power will vary widely from sanple to sauple. If, however, an enclosure is occupied then the signal will exhibit less variation.