Five basic ideas are identified, the combination of which consitutes radar. A clear distinction is then made between this combination of ideas, the contemporary technology from which it grew, and the contemporary scientific knowledge on which it was based. The mainstream of the development of radar is traced in a sequence of related events from 1922 to 1941. The technical problems encountered and the solutions employed in the first radar development are outlined in some detail. Two sidestreams of radar development are identified. The relationships among the three streams are discussed.