This is the first detailed assessment of the development and implementation of social policy to deal with the problem of the `mentally deficient' in Britain between 1870 and 1959. Mathew Thomson analyses all the factors involved in the policy-making process, beginning with the politics of the legislature and showing how the demands of central government were interpreted by local authorities, resulting in a wide and varied distribution of medical, institutional, and community care in different parts of the country. The efforts of health professionals, voluntary organizations and the families themselves are considered, alongside questions about the influence of changing concepts of class, gender, and citizenship. The author queries the belief that the policy of segregation was largely unsuccessful, and reveals a hitherto unrecognized system of care in the community. He reframes our understanding of the campaign for sterilization and examines why British policy-makers avoided extremist measures such as the compulsory sterilization introduced in Germany and parts of the US during this period. Thomson shows that the problem of mental deficiency cannot be understood simply in terms of eugenics but must also be considered as part of the process of adjusting to democracy in the twentieth century.