Microwave passive direction-finding technology has evolved from radar and earlier methods in a subtle way; we still trace this development from its rotary antenna and radar receiver origins to modern monopulse methods developed to provide sophisticated DF accuracy improvements. The evolution took place in answer to limitations in radar angle detection techniques that became evident as radar systems became widely developed. Problems included undesired lobe pickup, spurious responses to signals, return "glint," and limited dynamic range. First, solutions were to construct antennas with highly directional properties (very high front-to-back lobe responses) and multichannel receivers with special characteristics leading to the development of the relatively complex monopulse concept as we know it. Monopulse, however, was not the final result. Many radars designed for long-range surveillance, navigation, weather status, and ship positioning still use a single-channel antenna and receiver for simplicity and low cost. In situations in which well-trained operators are available, environments are light, and low cost is essential, nonmonopulse radars and associated single-channel DF receivers are frequently used. In situations in which high precision and immunity to jamming are of primary importance, monopulse becomes the technique of choice. As a consequence, the technology of passive direction finding must detect and recognize both monopulse and rotating radar techniques.