Mental health sciences witnessed a paradigm shift in the late twentieth century through the influence of three research themes. First, a focus on biology emerged following changes in psychiatric nomenclature and psychopharmacology. Second, advances in neuroscience provided heretofore unseen insights on the relationship between neural and information-processing functions, paving the way for a clinical neuroscience approach to mental illness. Third, the school of developmental psychopathology emerged, based on the recognition that most chronic mental illnesses have their roots in childhood. The current chapter, which focuses on the biology of childhood mental disorders, integrates these three themes. Given the breadth of work in each area, let alone the combination of the three, this chapter cannot provide a comprehensive review. Rather, I summarize major themes while providing illustrative examples from research on specific disorders.