This paper examines the historical context of the policy decision of the (then) DHSS in July 1974 to establish Regional Secure Units with an initial provision for 1,000 places. A brief examination of the history of the detention of the criminally insane and the setting up of the county asylums is followed by an examination of the various problems faced by the authorities concerned with the care of the criminally insane and the mentally ill in general in the 1960s. The paper examines the different streams of influence and power that converged upon this solution: government, special hospitals, public inquiries, unlocking of hospital wards, criminal law, DHSS and the Home Office, judges, voluntary bodies, prisons, psychiatrists and the official government reports known as the Glancy and the Butler Reports. The paper seeks to explain the policy decision to build regional secure units as a dynamic outcome arising from the confluence of opportunities, participants and solutions: a policy formation model put forward by March and Olsen (1976).