Worldwide, cannabis is the most commonly used illicit substance. In the United States, 42% of persons over age 12 have used cannabis at least once in their lifetime, 11.5% have used within the past year, and 1.8% have met diagnostic criteria for cannabis abuse or dependence within the past year. Among individuals who have ever used cannabis, conditional dependence (the proportion who go on to develop dependence) is 9%. This rate is lower than many other drugs of abuse, but it is nonetheless significant considering the high prevalence of cannabis use across the population. Children and young adults have substantially higher rates of conditional dependence, a concerning notion given the fact that in recent years a decade-long trend of decreasing cannabis use has reversed. Between 2007 and 2010, past month use among youth aged 12 to 17 increased from 6.7 to 7.4%, corresponding with a decrease in perception of risk over that same period. Altogether, there are 6600 new users of cannabis every day in the United States.