Provides an overview of the origins of child psychiatry and discusses several lines of epidemiological evidence that have fundamentally changed views of adolescent mental disorders. These are (1) data that challenged a long-held reputation of adolescence as a phase in which pronounced changes in psychological functioning inevitably followed from the disruptive effects of puberty; (2) prevalence surveys of adult psychiatric disorder that emphasized the importance of adolescent-onset mental disorders; and (3) secular trends in the age of onset of mental disorders and their outcomes that have drawn attention to adolescent onset psychiatric disorder. Although adolescent onset mental disorders are arguably the most serious health problems of adolescence both in mortality and morbidity, clinical and public health responses have been few. The prospect of a stream of psychiatry partnering adolescent medicine could do much to facilitate development of a preventive orientation in psychiatry and bridge the gulf between child and general psychiatry.