LPI radar is a system that consists of a radar and ES system. Its performance depends on both components. An LPI performance factor is derived and applied to several examples. Operational LPI radars are described. A digital LPI radar detector is described and test results presented. A recent book on LPI radar received a number of somewhat critical reviews that were published in Systems magazine. Although these reviews made a number of excellent points, several questioned the existence of LPI radar. The objectives of this article are to provide a sound technical basis for determining LPI properties of radar, examine several radars that have LPI qualities, and then to describe digital LPI radar detector designed and tested against the PILOT Mk2 radar. LPI radar is a system that represents a confluence between radar and electronic support (ES) technology. The objective of LPI radar is clear; that is, to escape detection by the ES receiver. However, the capability of the LPI radar system depends upon the characteristics of both the radar and ES receiver. To understand LPI radar one must first understand the nature of ES receivers. The functions performed by tactical ES receivers are immediate interception, identification, and location of both friendly and hostile emitters. Location requires interception in the emitter's sidelobes. Identification is performed by comparing the intercepted signature against the signatures contained within its threat library (Schelher, 1999). The metric that establishes the quality of interception is called the probability of intercept (POI) and is generally included in its specifications (Schleher, 1999). From the above discussion it is evident that radar might have LPI properties with respect to one type, but not to another type of ES system. This situation can be clarified if the claim of radar being LPI radar is accompanied by a description of the various types of ES systems for which this claim applies. However the LPI radar can never completely escape detection since there is always a minimum range (R/sub ES/) between the ES system and radar where the interception receiver detection threshold is exceeded.