Most mental and substance use disorders begin during childhood and adolescence and are the leading cause of disability in this population. Prenatal and postnatal genetic, familial, social, and environmental exposures interact to influence risk for mental disorders and trajectories of cognitive development. Efforts to advance prevention and implement early interventions to reduce the burden of mental disorders require a global research workforce, intersectoral cooperation, attention to environmental contexts, and the development and testing of evidence-based interventions. The authors describe challenges and resources for building mental health research capacity that stands to influence children's mental health outcomes around the globe.