Preventive detention is, together with life imprisonment, the harshest sanction in German criminal law. In the form of a “measure for improvement and security” of indeterminate duration, preventive detention potentially may be enforced until the death of the offender. Such a measure may be imposed together with a term of imprisonment on offenders regarded as dangerous and implemented after the fixed term of imprisonment has been served. The history of this provision goes back to a Nazi law against habitual criminals that was enacted in 1933 and retained after the end of the Second World War. In the context of the debate about the new criminal laws in the 1960s preventive detention was severely criticised. The result was that in 1969 both the formal and material requirements for the imposition of preventive detention were made more restrictive. This led, in turn, to it being imposed far less frequently. While in the 1960s more than 200 people were sentenced to preventive detention annually, by the early 1990s this figure was fewer than 40 a year. There was an equivalent reduction of the number of people in prison on preventive detention on a given day, from around 1500 at the beginning of the 1960s to fewer than 200 in the 1980s, that is, about 0.3 per cent of the total prison population.