This article reviews the role of developmental epidemiology in the prevention of child and adolescent mental disorders and the implications for systems of support. The article distinguishes between universal or primary prevention, which operates at the level of the whole community to limit risk exposure before the onset of symptoms, and secondary or targeted prevention, which operates by identifying those at high risk of developing a disorder. It discusses different aspects of time as it relates to risk for onset of disease, such as age at first exposure, duration of exposure, age at onset of first symptoms, and time until treatment. The study compares universal and targeted prevention, describing the systems needed to support each, and their unintended consequences.