This article reviews the literature on mood disorders and sleep disorders among children and adolescents. Research suggests that sleep plays an important role in the development, progression, and maintenance of mood disorder symptoms among children and adolescents. Sleep problems as early as maternal perinatal insomnia may predict and predate depression among youth. Children and adolescents who develop comorbid mood disorders and sleep problems represent a particularly high-risk group with more severe mood episode symptoms, higher rates of self-harm and suicidality, and less responsivity to treatment. Treatment research supports the idea that sleep problems can be improved through behavioral interventions.