This is the first account of the prison in the Weimar Republic (1918–33), set in the context of the evolution of German social policy. In the early years, the Weimar prison was characterized by hunger, overcrowding, and conflict. At this time, leading officials agreed on a new approach to imprisonment, influenced by the demand for the ‘incapacitation of incorrigibles, reformation of reformables’. This principle was championed by the modern school of criminal law, designed to replace traditional policy based on deterrence and uniform retribution. The policy of reform and repression shaped the Weimar prison. Most prison officials supported the indefinite confinement of ‘incorrigibles’. While this did not become law, many prisoners classified as ‘incorrigible’ (increasingly after ‘objective’ examinations) received worse treatment than others, both in prison and after their release. Regarding the ‘reformables’, some institutions introduced measures aimed at prisoner rehabilitation. But such policies were not fully implemented in other prisons, not least because of resistance by local prison officials. During the collapse of the Weimar Republic in the early 1930s, measures aimed at rehabilitation, only just introduced, were cut back again. By contrast, the repression of ‘incorrigibles’ was pursued with even more vigour than before, an important legacy for Nazi penal policy.