Background: Preventing externalizing problems in children is a major societal concern, and a great number of intervention programs have been developed to this aim. To evaluate their preventive effects, well-controlled trials including follow-up assessments are necessary. Methods: This is a systematic review of the effect of prevention programs targeting externalizing problems in children. The review covered peer reviewed publications in English, German, French, Spanish and Scandinavian languages. Experimental studies of standardized programs explicitly aiming at preventing externalizing mental ill-health in children (2-19 years), with outcome assessments at =6 months post intervention for both intervention and control groups, were included. We also included long-term trials with consecutive observations over several years, even in the absence of follow-up =6 months post intervention. Studies of clinical populations or children with impairments, which substantially increase the risk for mental disorders, were excluded. Results: Thirty-eight controlled trials assessing 25 different programs met inclusion criteria. Only five programs were supported by scientific evidence, representing selective parent training (Incredible Years and Triple-P), indicated family support (Family Check-Up), and school-based programs (Good Behavior Game, universally delivered, and Coping Power, as an indicated intervention). With few exceptions, effects after 6-12 months were small. Long-term trials showed small and inconsistent effects. Conclusions: Despite a vast literature, the evidence for preventive effects is meager, largely due to insufficient follow-up post intervention. Long-term follow up assessment and effectiveness studies should be given priority in future evaluations of interventions to prevent externalizing problems in children.