The main purpose of the present study was to explore how social competence reduces behavioral problems. Based on previous findings, we assume that increased social competence can be regarded as a mediating factor in reducing behavior problems. All participants (children and adolescents, n = 112) received an intervention intended to increase social competence: Aggresion Replacement Training (ART). Social competence and problem behavior were assessed twice before the ART intervention and then twice afterwards. Both measures improved following the training period, but no changes occurred during the pre-training period. Further, behavioral problems continued to reduce notably in follow-up probes after the training period. More detailed analyses indicate that in youngsters, increased social competence (e.g., improved self-control and cooperation) mediates the effect of ART on behavioral problems, but important moderating factors (e.g., age, individual levels of social competence, and problem behavior) need to be taken into consideration. (Contains 5 tables and 7 figures.)