The present meta-analytic review explored long-term treatment effects in children and adolescents, age ranging from 2 to 17 years, treated because of conduct problems. The presumed mediators were examined. Various treatments, in all 56 studies including 2589 participants, were included. 32 studies involved an untreated control condition, whereas 24 studies did not. The overall weighted effect size (ES) in conduct problems after termination of treatment was a reduction of ES = 0.08 for a mean follow-up period of 8.9 months. Long-term changes on the presumed mediator showed a small deterioration of ES = −0.06. The moderator analysis confirmed that treatment effects in conduct problems and mediators were related. Changes in conduct problems from post-treatment to follow-up were larger in studies with cognitive behavior therapy (CBT) or a combination of behavior therapy (BT) and CBT compared to BT and family therapy. Individual treatments resulted in larger changes in aggressive behaviour as compared to group treatments or a combination of these. Treatment effects for both conduct problems and the presumed mediators seem to last, but changes were small. Few studies included teenagers and knowledge of lasting treatment effects is limited for older children including CBT treatments. More knowledge of the mediators involved in treatment is needed, in particular for cognitive and family therapeutic interventions.