Objective: To critically review cannabis research during the past 10 years in relation to rates of use, behavioral problems, and mental disorders in young people. Method: Studies published in English between 1994 and 2004 were identified through systematic searches of literature databases. The material was selectively reviewed focusing on child and adolescent data. Results: In the 27 years between 1976 and 2002, approximately half of all 12th graders had been exposed to cannabis in the United States. There is growing evidence that early and regular marijuana use is associated with later increases in depression, suicidal behavior, and psychotic illness and may bring forward the onset of schizophrenia. Most of the recent data reject the view that marijuana is used to self-medicate psychotic or depressive symptoms. Research on treatment is very limited. Conclusions: Research on the mental health effects of cannabis has increased dramatically. Although doubts still remain about the role of cannabis in the causation of juvenile psychiatric disorder, the weight of the evidence points in the direction of early and regular cannabis use having substantial negative effects on psychosocial functioning and psychopathology.