RAJAR (Radio Joint Audience Research Limited) was established in 1992 to operate a single audience measurement system for the radio industry in the United Kingdom. RAJAR is jointly owned by the British Broadcasting Corporation (BBC) and the RadioCentre (Commercial Radio's trade body). Prior to this, the BBC and RadioCentre's predecessor (The CRCA) carried out their own measurements independently.

The methodology is based on a paper diary, which is filled in on a quarter-hour basis for one week by a representative sample. In a year, around 130,000 people complete a RAJAR diary. The research itself is carried out by contractors, currently Ipsos MORI.

There has been controversy over the collection methods, with the former owner of TalkSport, Kelvin MacKenzie, complaining that the diary method consistently under-reported his station's audience. The High Court threw out his lawsuit against RAJAR - the judge dismissing the central plank of his case as "impossible to see".

RAJAR has conducted extensive testing of electronic devices that capture listening, either by picking up encoded signals within station transmissions or by matching captured audio against a database of all transmissions. This would allow measurement to capture both 'conscious' and 'unconscious' listening and would theoretically end listeners attributing their listening to the wrong station. However, RAJAR was not convinced that the devices and the method were ready for use on the main survey and a new two-year contract using diaries was awarded to Ipsos MORI in May 2006, to begin in 2007.