From 1939 to 1945, Hitler sent the bombers of the Luftwaffe to attack military and civilian targets throughout the United Kingdom. The Royal Air Force was at first outnumbered, and had to find ways to rapidly locate and identify all aircraft approaching and entering British airspace. Most people know that radar was used to guide RAF Spitfires and Hurricanes to their targets in the Battle of Britain, but only a few recall that there was another method of spotting and identifying aircraft crossing the country. Members of the mainly-voluntary Observer Corps (OC) were on duty day and night, watching and listening in all weathers to identify and report the location of aircraft. During 1941, they were to gain the distinction of becoming the Royal Observer Corps. This was a remarkable recognition for a voluntary organisation, whose first operational Groups of Observer Posts were first formed in 1925.

The 1938 Munich crisis had added extra emphasis to the need for effective air defences, and by 1934 the Corps had begun spreading throughout the United Kingdom. Posts were organised into clusters of two to four Posts, reporting to Group centres, which passed information through Fighter Group and Sector Operations Rooms up to Fighter Command. The same system was to stay in being up into the 1960s, although details changed to keep pace with changing technology and new demands upon the system. By 1939, most of the mainland United Kingdom was covered by the Observer Corps Groups, although the 'Back Areas' (Highlands, south west Scotland, western Wales and Cornwall) were only poorly-equipped. Northern Ireland and the Isle of Man had no Observer Posts at all.

The Corps' finest hour is probably its contribution to the Battle of Britain in 1940, when the reliable eyes of the Observers supplemented the then-crude radar network. The fine June weather made it possible for the Observers to keep a close watch on the aerial battle. At one stage, the Luftwaffe damaged the radar stations, but the Observers kept in action despite occasional strafing and bomb damage. The Home Chain and Home Chain Low radars were almost useless inland, so the Observers were vital to successful interceptions.

Surprisingly, the Germans never developed any kind of instrument to help with aircraft observation, and their own 'Flugmeldedienst' never had the same degree of professionalism as was achieved by the Observer Corps. On April 9th 1941, this professionalism and Blitz service was rewarded by H.M. King George VI approving the change of title to 'Royal Observer Corps'. This link with royalty remained throughout the rest of the life of the Corps, H.M. Queen Elizabeth II having remained as patron of the Corps up to its stand-down in 1992.