Recidivism (repeated offenses after conviction) for most serious crimes is high. According to a U.S. Department of Justice report,46 of the more than 100,000 persons released from prisons in 11 states in 1983, an estimated 62.5% were rearrested for a felony or serious misdemeanor within three years. A RAND report which looked at prison populations generally concluded that “released inmates, as a group, pose a very serious threat to public safety, but we cannot predict with useful accuracy which inmates will recidivate.” In California, the state Department of Justice concluded that “sex offenders do not differ significantly in terms of overall recidivism from most other types of offenders.” In understanding recidivism among child molesters, an important question is what level of recidivism can be expected in the absence of treatment, and whether any characteristics of offenders have been identified that predict an above- or below-average risk of recidivism. To address these topics, we must first consider the problem of measuring recidivism.