Epidemiology, the study of patterns of disease distribution in time and space, can help to improve mental health services for children and adolescents by increasing understanding of the causes, development, and course of psychiatric disorders. For the purpose of service delivery, epidemiologic research on child psychopathology can provide information on need for services, availability of services, and effectiveness of services. For both scientific and planning purposes, epidemiologic research can inform us about (1) the developmental course of psychiatric disorders during childhood and adolescence, (2) the effect of psychiatric disorder on the course of normal childhood development, and (3) the effect of childhood development on the developmental course of psychiatric disorder. Information about prevalence and incidence is useful for planning primary, secondary, and tertiary prevention and treatment services. The same information is scientifically useful to the extent that it helps to answer questions about causation, course, and outcome. However, one aim of this paper is to show that epidemiology is about a great deal more than rates of disorder.