As the field of radar is very large, the treatment in the paper is necessarily brief, and is mainly confined to techniques and systems of current interest. All radar systems are subject to fundamental performance constraints stemming from the range, range rates and angle-measurement processes which are basic to radar. Most modern radar systems of interest are subject to further constraints arising from the environment in which they operate. The choice of the radiated carrier frequency affects many aspects of radar performance, including aerial directivity, the ability to distinguish target radial velocity, the target echoing area, attenuation in the atmosphere and clutter echoes. Highly directional aerials provide the basis for radar angle measurement and significant recent advances have been made in this field. Transmitters, including high-power transmiters and low-power solid-state microwave oscillators and amplifiers, are discussed. Radar receiving systems have been strongly influenced by recent advances in solid-state technology. Closely associated with the basic receiving function is the signal processing needed to achieve an acceptable approximation to matched filtering of the radar-echo signal. Radar-display technology continues to be dominated by the cathode-ray tube, however, the data presented for display are increasingly subject to preprocessing, often by digital computer, prior to display to the radar user. The use of computers in association with radar, initially to facilitate information display, has been broadened to more fully adaptive systems, in which the operating parameters of the radar are continuously adjusted to match the immediate requirements of the task in the light of information already obtained. By reradiating a pseudoecho on a frequency different from that of the radar, co-operative targets may be detected without any problems of clutter. Such radar operation is termed secondary radar. The paper concludes with brief description of air-surveillance radar, radar in guided weapons, radar in aircraft navigation, radar in airborne air defence, radar on the battlefield, miniature radars, radar meteorology and over-the-horizon radar.