Background: Social and emotional wellbeing is a crucial resource for all children. Yet high-quality epidemiological studies suggest that 12.7% of children will experience a mental disorder. Methods: We used systematic review methods to identify 113 randomized controlled trials and six systematic reviews on effective interventions for preventing and treating the most common childhood mental disorders. Results: We found effective prevention interventions for anxiety disorders, attention-deficit/hyperactivity disorder, oppositional defiant and conduct disorders, depression, eating disorder and posttraumatic stress disorder. In addition to identifying useful treatments for all of these disorders, we also found effective treatments for autism spectrum, obsessive-compulsive and bipolar disorders as well as schizophrenia. Conclusions: Mental disorders, which typically start in childhood, cause significant distress and impairment for children. Beyond the hardships they cause for children and families, there are also significant avoidable costs for society as a whole. By implementing effective prevention interventions and treatments for these concerns, it is possible to avert unnecessary hardships for young people, their families and their communities.