Cannabis dependence is controversial. What are the health and behavioral risks of becoming cannabis-dependent? What counseling approaches have been tested with adults and adolescents, and how effective are they? What are the arguments for legalization, regulation, or prohibition? Looking back and toward the future, what do we know and what do we need to learn? This state-of-the-science review sets out to answer all such questions, beginning with an historical examination and moving into diagnosis, classification, epidemiology, public health, policy, issues relating to regulation and prohibition, and evidence-based interventions.