Following a review of the general characteristics of effective treatment, the article profiles the Regional Treatment Centre Sex Offender Treatment Program (RTCSOP) in Ontario, Canada. The RTCSOP has focused on treating high-risk/high-treatment-need sexual offenders since its inception in 1973. An evaluation study that assessed program graduates over 10 years compared with a matched control group concluded that the RTCSOP was particularly effective in reducing the percentage of program participants in repeated sex offenses after treatment. The article also reviews the effectiveness of the California Department of Mental Health's Sex Offender Treatment and Evaluation Project (SOTEP). Under this program, clients attend at least 1 year of a 2-year treatment program while incarcerated, requiring 40-50 hours per week of participation. Extensive community supervision, including a requirement to attend treatment, can last for up to 2 years following release. Based on a review of the evaluation findings for this program, in addition to the evaluation findings of the Ontario treatment program, this article offers a number of tentative conclusions. First several studies suggest that cognitive-behavioral treatment with sex offenders can be effective in reducing their sex offenses compared to nontreatment conditions. Still, given the limited number of well-controlled, longitudinal outcome studies that use large groups of subjects, definite conclusions cannot yet be drawn. The authors argue, however, that there is sufficient evidence to conclude that cognitive behavioral treatment geared to the principles of risk/need/responsivity can be effective in reducing recidivism in sex offenders. The authors offer suggestions as to how treatment providers might become more actively involved in research. Also discussed is the possibility that harm-reduction models might be usefully applied to the field of sex offender treatment. 78 references