An array antenna can enlarge the capability of a radar system by contributing beam-pointing agility as well as providing a way of handling large power. The fundamental theory of such antennas was outlined in IEEE Spectrum exactly four years ago.1 Advances in the art have been so impressive that this month's Proceedings of the IEEE is a special issue on Electronic Scanning. The present article discusses the principles of operation and the most general applications of one very important class of array antennas.