During the next two years HMSO will be publishing a three-volume official history of British intelligence during the Second World War. In this talk, the author F. H. Hinsley , Master-elect of St John's College, Cambridge, and Professor of the History of International Relations, argues that whilst intelligence operations are interesting for their own sake, the significance for historians is to unravel what actual effect they had on the course of the war - a task not without its complexities,