Abstract: A system and method for continuously monitoring the shielding effectiveness f an electromagnetically shielded enclosure is disclosed including an RF transmitter positioned remote from the shielded enclosure. RF signals emitted from the remotely positioned transmitter are received by both an enclosure receiver positioned inside of the enclosure and simultaneously by a reference receiver having its antenna positioned outside of the shielded enclosure. These two received signals are mixed so as to produce IF signals which are subsequently forwarded to a synchronous detector which determines the ratio of the two signal levels by comparing their strengths. This ratio is indicative of the enclosure's electromagnetic shielding effectiveness. If the effectiveness drops below a predetermined or threshold limit, such is determined by a comparator positioned within the enclosure and an alarm may be sounded so as to indicate that the shield is failing.