The article reports a systematic review of controlled outcome evaluations of psychosocial and organic sexual offender treatment. A comprehensive search of the literature in five languages revealed 80 independent comparisons between treated and untreated groups of sexual offenders ( N = 22,181). The majority of studies confirmed a positive treatment effect. Overall, 11.1% of treated offenders and 17.5% of controls showed sexual recidivism (37% difference). Findings for violent and general recidivism were similar. Studies on surgical castration showed the strongest effect; however, this was confounded with methodological and offender characteristics. Hormonal medication, cognitive-behavioural, and behavioural approaches also revealed a positive effect. Non-behavioural treatments did not show a significant impact. Other moderators such as small sample size, authors' affiliation with the program, program completion versus dropout, or type of outcome measure had a significant impact. Methodological study characteristics explained the largest proportion of effect size variance. Overall, findings are promising but more differentiated evaluations of high quality are needed.