Cannabis dependence is controversial. Does it occur or is it a myth put forth by those who oppose legalisation? What are the signs of cannabis dependence? How many people are affected? What are the health and behavioural risks of becoming cannabis-dependent? What counselling approaches have been tested with adults and adolescents, and how effective are they? What are the arguments for legalisation, 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 those 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.