To mark its centenary, the British Security Service, MI5, has for the first time opened its archives to an independent historian. The book reveals the precise role of the Security Service in 20th-century British history, from its foundation in 1909 to its present roles in counterespionage and counterterrorism.

Christopher Andrew is Professor of Modern and Contemporary History and Chair of the Faculty of History at Cambridge University. He is also chair of the British Intelligence Study Group, Co-Editor of Intelligence and National Security, former Visiting Professor at Harvard, Toronto and the Australian National University, and a regular presenter of BBC Radio and TV documentaries. His thirteen previous books include The Mitrokhin Archive volumes 1 and 2, and a number of path-breaking studies on the use and abuse of secret intelligence in modern history.