Findings from meta-analyses in Germany, Canada, and the United States illustrate variables that moderate the outcome of offender treatment, including treatment integrity and intensity, offender characteristics, and training and motivation of treatment personnel. The comparison between countries shows generally similar overall treatment effects. Both content-related and methodological issues in correctional treatment and rehabilitation are addressed. Meta-analyses of the impact of psychotherapy on treatment outcomes are reported, the success of treatment and rehabilitation programs based on a cognitive-social learning approach and on appropriate offender treatment is discussed, and principles of "what works" in the field of corrections are described. 135 references