This dissertation studies the way that Britain's intelligence services changed priorities from the Second World War to the early Cold War. It stretches from the point when the Soviet Union entered the Second World War as Britain's ally in 1941, to the moment a decade later in 1951, when the Cold War had set in and Moscow was the bitter enemy of the west. Using recently declassified Security Service (MIS) records, it examines how Britain's intelligence services met the massive transition from World War to Cold War. It reveals a variety of subjects previously undocumented in the secondary historical literature, such as MIS's concerns after the Second World War with terrorism emanating from the Middle East. The dissertation is an attempt to rescue intelligence from historical obscurity and place it in its justified position: as a central component in the process of political decision-making in Britain. As well as offering new historical insights, it provides useful lessons for governments and intelligence agencies at the start of the twenty-first century. The dissertation shows that many of the issues facing intelligence agencies at the start of the twenty-first century were, in fact, faced by the British intelligence community half a century ago.