The United States prison inmate population surpassed 1,000,000 in 1994, quadrupling the number from 20 years earlier. The event sparked renewed debate over the efficacy of imprisonment as a primary policy for ensuring public safety. The article finds that imprisonment has been a sound public investment in that the public has averted greater social costs from crime than the costs expended for imprisonment. Recent estimates of the social costs of violent crimes suggest that marginal increases in confinement remain cost-effective even in the face of possible diminishing returns as lower-rate offenders are incarcerated. The article recommends greater attention to the relative merits of deterrence and incapacitation factors for various subgroups of offenses and offenders, and encourages studies on the effectiveness of community corrections. The article also examines several arguments against imprisonment and suggests ways to recast some of them into more persuasive constructs. Tables, references