Radar in World War II greatly influenced many important aspects of the conflict. This revolutionary new technology of radio-based detection and tracking was used by both the Allies and Axis powers in World War II, which had evolved independently in a number of nations during the mid 1930s. At the outbreak of war in September 1939, both the United Kingdom and Germany had functioning radar systems. In the UK, it was called RDF, Range and Direction Finding, while in Germany the name Funkmeß (radio-measuring) was used, with apparatuses called Funkmessgerät (radio measuring device). By the time of the Battle of Britain in mid-1940, the Royal Air Force (RAF) had fully integrated RDF as part of the national air defence.

In the United States, the technology was demonstrated during December 1934. However, it was only when war became likely that the U.S. recognized the potential of the new technology, and began the development of ship- and land-based systems. The U.S. Navy fielded the first of these in early 1940, and a year later by the U.S. Army. The acronym RADAR (for Radio Detection And Ranging) was coined by the U.S. Navy in 1940, and the term "radar" became widely used.

While the benefits of operating in the microwave portion of the radio spectrum were known, transmitters for generating microwave signals of sufficient power were unavailable; thus, all early radar systems operated at lower frequencies (e.g., HF or VHF). In February 1940, Great Britain developed the resonant-cavity magnetron, capable of producing microwave power in the kilowatt range, opening the path to second-generation radar systems.

After the Fall of France, Britain realised that the manufacturing capabilities of the United States were vital to success in the war; thus, although America was not yet a belligerent, Prime Minister Winston Churchill directed that Britain's technological secrets be shared in exchange for the needed capabilities. In the summer of 1940, the Tizard Mission visited the United States. The cavity magnetron was demonstrated to Americans at RCA, Bell Labs, etc. It was 100 times more powerful than anything they had seen. Bell Labs was able to duplicate the performance, and the Radiation Laboratory at MIT was established to develop microwave radars. The magnetron was later described by American military scientists as "the most valuable cargo ever brought to our shores".

In addition to Britain, Germany, and the United States, wartime radars were also developed and used by Australia, Canada, France, Italy, Japan, New Zealand, South Africa, the Soviet Union, and Sweden.

Research leading to RDF technology in the United Kingdom was begun by Sir Henry Tizard's Aeronautical Research Committee in early 1935, responding to the urgent need to anticipate German bomber attacks. Robert A. Watson-Watt at the Radio Research Station, Slough, was asked to investigate a radio-based "death ray". In response, Watson-Watt and his scientific assistant, Arnold F. Wilkins, replied that it might be more practical to use radio to detect and track enemy aircraft. On 26 February 1935, a preliminary test, commonly called the Daventry Experiment, showed that radio signals reflected from an aircraft could be detected. Research funds were quickly allocated, and a development project was started in great secrecy on the Orford Ness Peninsula in Suffolk. E. G. Bowen was responsible for developing the pulsed transmitter. On 17 June 1935, the research apparatus successfully detected an aircraft at a distance of 17 miles. In August, A. P. Rowe, representing the Tizard Committee, suggested the technology be code-named RDF, meaning Range and Direction Finding.

In March 1936, the RDF research and development effort was moved to the Bawdsey Research Station located at Bawdsey Manor in Suffolk. While this operation was under the Air Ministry, the Army and Navy became involved and soon initiated their own programs.

At Bawdsey, engineers and scientists evolved the RDF technology, but Watson-Watt, the head of the team, turned from the technical side to developing a practical machine/human user interface. After watching a demonstration in which operators were attempting to locate an "attacking" bomber, he noticed that the primary problem was not technological, but information management and interpretation. Following Watson-Watt's advice, by early 1940, the RAF had built up a layered control organization that efficiently passed information along the chain of command, and was able to track large numbers of aircraft and direct interceptors to them.