Since their first appearance in the mid-nineties, gunshot detection systems have been deployed by law enforcement agencies attempting to reduce the illegal use of firearms in highcrime urban areas. The use of this technology has spread as agencies seek to obtain better real-time geospatial information on gunshots in public spaces, although the performance and reliability of these systems is still an active research area. This paper reviews the basic physics of gunshots and the sound production mechanism of gunfire, summarizes the history of gunshot detection research, and surveys the published technical and performance information regarding commercial systems.