This text defines small-aperture radio direction-finding and identifies its unique niche in the passive radiolocation discipline. This book provides a discussion of the functional elements of radio direction-finding (DF) and a definition of small-aperture DF based on linear wavelength criteria. It introduces the topic of signal direction-of-arrival measurement techniques using amplitude, phase and time. The book also presents the history and evolution of the discipline as a whole and should appeal to engineers in the areas of radio, antennas, radar and electronic warfare.