Arising from First World War attempts to develop a method of tracking aircraft, the Observer Corps started experimentally in southern England in the mid-1920s. Successful trials led to the expansion of the Corps and in 1937 a group was forming in the Scottish area with headquarters planned to be in Edinburgh. At the same time, outlying posts were being set up which would observe aircraft movements, passing the relevant details of aircraft type, formation size, height and direction of flight to the Group Headquarters. Changing plans meant that when the group covering South-East Scotland was eventually formed in 1938, its headquarters would be in Galashiels, not Edinburgh and would be numbered No 31 Group. This group was responsible for observer posts throughout the Lothian and Border counties, with a number of posts in East Lothian at the following locations: North Berwick, Athelstaneford, Aberlady, Dunbar, Innerwick, Garvald, Humbie and Tranent.

The posts of the Observer Corps (renamed the Royal Observer Corps in 1941) were organised in clusters, which were linked to a single plotter at the operations table at Group Headquarters. North Berwick (post A1), Athelstaneford (post A2) and Aberlady (post A3) formed a single cluster, as did Dunbar (post Bl), Innerwick (post B2) and Garvald (post B3). Humbie and Tranent formed separate clusters with posts outside East Lothian.

The Observer Corps was an essential component of the wartime air defence system. Initial early warning of approaching aircraft was provided by the coastal radar chain. Once aircraft passed inland, tracking thereafter relied on the Observer Corps. The two organisations complemented each other well, with the Observer Corps providing additional information on aircraft types, formation sizes and flying height, information that was either unavailable or not always reliable from the radar system. On numerous occasions the aircraft identifications provided by the Observer Corps posts were doubted by Fighter Command, but were ultimately proven correct. It is sometimes forgotten that the Observer Corps provided an invaluable service in tracking friendly, as well as hostile, aircraft and in assisting with warning lost aircraft of high ground. This latter facility was provided by the provision of flares, codenamed Granite, which could be ignited in poor visibility as a warning to aircraft in the vicinity. Throughout Britain, the introduction of Granite in late 1943 saw a substantial reduction in flying accidents. The significance of such contributions is heightened by the fact that personnel of the Observer Corps were largely part-time volunteers, who had to fit their observer duties in with other work.

The East Lothian posts were to play an important part in the history of the Observer Corps, being the first to plot enemy aircraft anywhere in Britain. The said aircraft was a Heinkel He 111 on a reconnaissance mission on the morning of 16 October 1939, prior to the attack on ships of the Royal Navy in the Firth of Forth carried out by Junkers Ju 88s of KG 30. Air Chief-Marshal Sir Hugh Dowding, Air Officer Commander-in-Chief of Fighter Command acknowledged the contribution of the Observer Corps to the successful action that day in a message in which he noted: "I am very pleased with the way in which the Observer Corps acquitted themselves in this, the first action of the war in which they are able to take part... In the circumstances I consider the Observer Corps operated with great efficiency." Twelve days later the East Lothian observer posts were again to see action as they tracked the combat which resulted in the shooting down of the Heinkel He 111 which crashed on the north face of Witches Knowe, Longnewton Farm, near Humbie and the location of post J1.

One member of the Royal Observer Corps was also to play an important role in the organisation, but outwith East Lothian itself. Alexander Thomson was a farmer from Samuelston who was also a part-time member of the ROC. When a request was issued in 1944 for volunteers to sail with the D-Day invasion fleet to provide aircraft recognition services for the anti-aircraft gunners on the ships, Mr Thomson and Mr Falgate from East Lothian were two of the 1,376 volunteers. Of these, eventually 796 were selected and enrolled in the Royal Navy. Alexander Thomson and Mr Falgate served with a US ship in May and June 1944 for a total period of around a month. The American gunners noted that they were very good at aircraft recognition and their services were greatly appreciated, since they prevented the shooting down of numerous friendly aircraft!

With the end of the war the role of the Royal Observer Corps in tracking hostile aircraft had clearly come to an end, and the corps was stood-down on 12 May 1945. The corps was reactivated in January 1947, following the realisation of its important role within the air defence organisation. However, with the development of jet aircraft, and the ever-increasing speed of such aeroplanes as the technology improved, the ability of the ROC to provide up-to-date information on aircraft movements was gradually reducing. Eventually, it was decided in 1957 that the aircraft reporting role should be abandoned (although it continued in some areas of England until 1965) and that underground posts should be built from which drifting fallout from nuclear explosions could be tracked. For this purpose, undergound posts were built in East Lothian at Aberlady, Athelstaneford, Cockburnspath (although actually near Oldhamstocks), Dunbar, Garvald, Humbie, North Berwick and Tranent. Although never tested in anger, these posts would have provided a vital function in a post nuclear-attack Britain, providing information on the location of nuclear detonations and on areas likely to be affected by fallout, making it possible to continue some form of central governmental control. This nuclear reporting role continued until the Royal Observer Corps was stood-down again in September 1991, following the end of the Cold War, with the organisation finally being completely disbanded in March 1992.