Abstract: We review scientific criteria for the minimally useful evaluation of psychosocial treatment for sex offenders. The Association for the Treatment of Sexual Abusers recently supported a meta-analysis (Hanson et al., 2002) of the effectiveness of psychological treatment for sex offenders. It was concluded that current treatments for sex offenders reduce recidivism. In this chapter, we reevaluate the evidence. Whereas the random assignment studies yielded results that provided no evidence of treatment effectiveness, Hanson et al. reviewed approximately a dozen others (called ‘incidental assignment’ studies), which yielded substantial positive results for treatment. Upon close inspection, we conclude that such designs involve noncomparable groups and are too weak to be used to draw inferences about treatment effectiveness. In almost every case, the evidence was contaminated by the fact that comparison groups included higher-risk offenders who would have refused or quit treatment had it been offered to them. We conclude that the effectiveness of psychological treatment for sex offenders remains to be demonstrated. Furthermore, we outline solutions that we think will lead to progress in the field of sex offender treatment.