Maladaptive aggression in adolescents is an increasing public health concern. Cognitive Behavior Therapy (CBT) is one of the most common and promising treatments of aggression. However, there is a lack of information on predictors of treatment response regarding CBT. Therefore, a meta-analysis was performed examining the role of predictors on treatment response of CBT. Twenty-five studies were evaluated (including 2,302 participants; 1,580 boys and 722 girls), and retrieved through searches on PubMed, PsycINFO and EMBASE. Effect sizes were calculated for studies that met inclusion criteria. Study population differences and specific CBT characteristics were examined for their explanatory power. There was substantial variation across studies in design and outcome variables. The meta-analysis showed a medium treatment effect for CBT to reduce aggression (Cohen'd = 0.50). No predictors of treatment response were found in the meta-analysis. Only two studies did examine whether proactive versus reactive aggression could be a moderator of treatment outcome, and no effect was found of this subtyping of aggression. These study results suggest that CBT is effective in reducing maladaptive aggression. Furthermore, treatment setting and duration did not seem to influence treatment effect, which shows the need for development of more cost-effective and less-invasive interventions. More research is needed on moderators of outcome of CBT, including proactive versus reactive aggression. This requires better standardization of design, predictors, and outcome measures across studies.