Given the recent surge of interest in Open Source Intelligence (OSINT), surprisingly little attention has been devoted to existing open source agencies or their historic role. As the first full length academic study of the BBC Monitoring Service, this thesis seeks to examine one of the earliest historic attempts to systematically manage and exploit publicly available, open source information for intelligence purposes. It utilises the Imperial War Museum’s rarely used collection of BBC Monitoring transcripts, in conjunction with traditional archival sources and oral interviews, to trace the origins, processes and institutional structure developed by the Monitoring Service during its formative years. This study further assesses the collection priorities of BBC Monitoring during World War Two, traces the historic flow of monitored material around the wartime Government and BBC, and establishes the institutional role of BBCM in the conduct of Britain’s war effort. Three case study chapters, focusing on the outbreak of war, Dunkirk and D-Day, particularly assess the process and detailed collection priorities of the Monitoring Service during key events throughout the war. This study thus makes a contribution to the historic picture of British intelligence during World War Two, and is bound to encourage future study of the BBC Monitoring Service and its archives. Overall, the BBC Monitoring Service is judged a historic success story. This thesis argues that this success can be attributed to three key qualities developed during the wartime period: trust, breadth, and adaptability. The organisation established a relationship of trust with the Government; developed and maintained a remarkable breadth of broadcast coverage; and showed a constant ability to adapt to both customer demands and changing strategic priorities.