Epidemiologic studies show that major depression is comparatively rare among children, but common among adolescents, with up to a 25% lifetime prevalence by the end of adolescence. Mania is much less common, with no more than a 2% lifetime prevalence by the end of adolescence. Developmental studies that include assessments of both hormonal changes and social changes through the pubertal transition are needed to investigate joint biological and environmental influences on the emergence of the gender difference in depression in puberty. Although subthreshold mood disorder symptoms are common, controversy exists about their clinical significance. This controversy is made more complex by methodologic uncertainties regarding inconsistent symptom reports obtained from parents, teachers, and children and by the pervasive existence of comorbidity. Retrospective reports about age of onset in adult studies suggest that at least 50% of youngsters with major depression and 90% of those with mania continue to have adult recurrences. These recurrences are mediated by adverse role transitions, such as truncated educational attainment and teenage childbearing, that typically occur before the time of initial treatment. Aggressive outreach and early treatment aimed at preventing the occurrence of adverse role effects might help decrease the persistence of child and adolescent mood disorders. Long-term follow-up studies are needed to resolve current uncertainties regarding nosology, methodology, and long-term treatment effects. Innovative epidemiologic research designs aimed at more quickly providing provisional information are also needed to advance understanding of long-term developmental processes.