The past twenty years have seen the rapid growth of a new branch of international history, the serious academic study of secret services or ‘intelligence history’ with its attendant specialist conferences and journals. Two main causes for this development can be identified. The first was conceptual, namely the increasing recognition that the study of international history was greatly impoverished by the reluctance of academic historians to address a subject which appeared capable of shedding considerable light upon the conduct of international affairs. Two leading historians underlined this during 1982 in a path-breaking collection of essays on the subject, suggesting that intelligence was the ‘missing dimension’ of most international history. The second development was a more practical one, the introduction of the Thirty Year Rule during the 1970s, bringing with it an avalanche of new documentation, which, within a few years, was recognized as containing a great deal of intelligence material. In the 1980s historians had begun to turn their attention in increasing numbers to the intelligence history of the mid-twentieth century. They were further assisted in their endeavours by the appearance of the first volumes of the official history of British Intelligence in the Second World War.