The aim of this article is to review recent research on the prevalence, antecedents and correlates of cannabis use in young adults in developed and developing countries. Cannabis is the most widely used illicit drug globally and its use appears to be increasing in developed and developing countries. In developed countries rebelliousness, antisocial behaviour, poor school performance, and affiliation with drug-using peers are risk factors for early and regular cannabis use. Similar antecedents are now being reported in developing countries. Dependence is an underappreciated risk of cannabis that affects one in six to seven adolescents who use cannabis in developed countries. Adolescent cannabis dependence is correlated with an increased risk of using other illicit drugs, symptoms of depression, and symptoms of psychosis. The plausibility of cannabis playing a contributory causal role has increased for symptoms of psychosis in longitudinal studies but remains contentious. In the case of other illicit drug use and mood disorders common causal explanations remain difficult to exclude. Early and regular cannabis use in adolescence predicts an increased risk of cannabis dependence which in turn predicts an increased risk of using other illicit drugs, and reporting symptoms of mood and psychotic disorders.