This is a history of the secret activities of the British government in response to threats to the nation's well-being and stability during the twentieth century. It is based on intensive and widespread research in private and public archives and on documents many of which have only recently come to light or been made available. The dangers perceived by the state have been manifold and various, coming from within and from abroad. Anarchists, fascists, socialists, communists, the IRA, trades-unionists and animal activists as well as spies, terrorists and saboteurs have been the subject of undercover investigation, along with almost every large-scale movement from suffragettes to campaigners for peace and nuclear disarmament. The author describes the methods and people employed, and the mixed nature of their results. The British state has always seen itself as civil and liberal, but as Dr Thurlow shows it has sometimes been far from open. The government has had many weapons at its disposal, from public order acts, censorship, internment and proscription on the one hand, to covert operations, infiltration and manipulation on the other. Yet when examined in the light of new evidence, the activities of the state are fully comprehensible only in terms of those who comprised it. The author shows the tensions among the departments (between MI5, MI6, SIS and the Special Branch, for example), and the crucial part played by individuals whose motives were often far from what the government supposed them to be. This is an at times disturbing, at others almost comical, but always fascinating account. It throws light on the inmost workings of the state, as well as on the movements and people subject to investigation and action.