New sentencing reforms imply greater criminal system costs. Policymakers need to know which type of offender and what length of sentence are likely to produce the largest reduction in crime, and the impact on size of prison population. This paper reports on research that attempts to answer these questions. Mandatory sentences can reduce crime as a result of incapacitation effects, but the increase in prison population may be unacceptably large. To reduce crime by half, every convicted offender would have to be imprisoned for five years. If only defendants who have a prior adult conviction are imprisoned, the crime-reduction effect is about half the effect produced by sentencing every convicted felon to prison. This analysis suggests the most efficient policy is to sentence all convicted felons to 1.2 years of prison, resulting in a 20 percent reduction in the crime rate, and raising the prison population 85 percent.