Purpose of Review: To summarize subjective and polysomnographically measured sleep disturbances in children and adolescents with psychiatric conditions, including anxiety, mood, and neurodevelopmental disorders and high-risk syndromes for psychosis. Evidence for the role of disrupted sleep in the onset and maintenance of each diagnostic category is considered.Recent Findings: Although subjective sleep complaints are common and transdiagnostic among youth with psychiatric disorders, evidence for disruptions in traditional polysomnographic measures (e.g., total sleep time) is inconsistent. However, physiological sleep abnormalities (e.g., delta or sigma power, rapid eye movement density) may characterize specific mental health conditions. Overall, sleep disturbances appear to be precursors to, and sustainers of, psychiatric symptoms in youth.Summary: Future studies may elucidate physiological sleep processes underlying subjective sleep complaints in youth with psychiatric disorders and would benefit from polysomnographic methodology standardization, diverse/representative samples, incorporation of circadian measures, and inclusion of polysomnography as an outcome in behavioral sleep intervention clinical trials.