This chapter describes selected features of cannabis epidemiology, with a focus upon recent evidence from field studies of cannabis dependence. Recent epidemiological evidence from the USA speaks to the question of how many individuals might need clinical services early in the cannabis dependence process. Among the 14 million current users, roughly 1.6-2.3 million users qualified as a recently active Diagnostic and Statistical Manual, (DSM-IV) cannabis dependence case according to the National Survey field assessment methods and DSM-IV diagnostic algorithm. Research on early-onset drug use and later increased risk of drug problems has encompassed investigations into early-onset cannabis use as a suspected determinant of later increased risk of cannabis dependence. Differences between cannabis users and non-users also complicate other inquiries into the health and social consequences of cannabis involvement. To date, epidemiology's contribution to research on prevention, intervention, amelioration, and control of cannabis dependence has been indirect.