This study reports on a meta-analysis of moral reconation therapy (MRT). Recipients of MRT included adult and juvenile offenders who were in custody or in the community, typically on parole or probation. The study considered criminal offending subsequent to treatment as the outcome variable. The overall effect size measured by the correlation across 33 studies and 30,259 offenders was significant (r = .16), indicating that MRT had a small but important effect on recidivism. Moderator analyses were conducted to detect the possible factors affecting the relationship between MRT and recidivism. Moderators included setting, age, gender, research design, sample size, type of recidivism, follow-up period, publisher, and year of publication. Moderator analysis demonstrated that MRT was more successful with adult than juvenile offenders in institutional settings as opposed to the community, and where researchers in the primary studies used randomization to allocate participants to either a treatment or control condition. The treatment effect size was greater when the type of recidivism used was rearrest rather than rearrest followed by conviction or reincarceration. The benefits of MRT were strongest with a relatively short follow-up period. MRT was more successful for relatively small samples and for large samples rather than medium-sized samples. The effect size was smaller for studies published by the owners of MRT than by other independent studies. The effect size was also smaller for studies published after 1999.