Sir Robert Alexander Watson-Watt, (born April 13, 1892, Brechin, Forfarshire [now Angus], Scotland—died December 5, 1973, Inverness, Inverness-shire), Scottish physicist credited with the development of radar in England.

Watson-Watt attended the University of St. Andrews and later taught at University College, Dundee. From 1915 to 1952 he held a number of government positions, beginning as a meteorologist working on devices for locating thunderstorms. In early February 1935, while heading the radio department of the National Physical Laboratory in Teddington, England, he wrote a memorandum to the British government in which he explained how radio waves could be used to detect aircraft. He quickly followed with an experimental demonstration. By July 1935 Watson-Watt was able to locate aircraft consistently at a distance of about 140 km (90 miles). His system grew into a series of radars called Chain Home, which typically operated at frequencies of 22–50 megahertz, which were much lower than radars developed in other countries prior to World War II. Watson-Watt justified his choice of a nonoptimal frequency for his radar with his often-quoted “cult of the imperfect,” which he stated as “Give them the third best to go on with; the second best comes too late, the best never comes.” In September 1938 the first of the Chain Home radars began 24-hour duty. By the time World War II began a year later, there were 18 radars defending the United Kingdom, and this number grew to 53 before the war ended in 1945. Chain Home radars are given much credit for the small Royal Air Force’s turning back the German Luftwaffe during the Battle of Britain in 1940. Watson-Watt was knighted in 1942.

Watson-Watt’s other contributions include a cathode-ray-tube direction finder used to study atmospheric phenomena, research in electromagnetic radiation, and other inventions used for flight safety.