Examines several issues on psychiatric diagnoses as risk factors for adolescent drug use and substance use disorders. The articles in this special section are discussed in terms of dynamic models of risk and protective factors, stages of research needed for establishing causal mechanisms in the etiology of drug use disorders, conceptualizing drug use and abuse in the context of the developmental organization of the individual, equifinality and multifinality in developmental pathways to drug use and abuse, and issues in conceptualizing drug use problems as mental disorders. Future directions for research on the etiology of problem drug use, guided by principles of developmental psychopathology and prevention science, are provided.