Marijuana is the most frequently used illicit drug in the United States, with approximately 14.8 million Americans over the age of 12 reporting past-month use in 2006. While marijuana use declined in the 1980s, its use among all youth--including college students--rose steadily in the 1990s. Prevention professionals report concern because marijuana may act as a "gateway" drug, serving as an introduction to the "drug scene" and additional types of drug use. In fact, studies have found that "nearly all adolescents who use illicit drugs other than marijuana also used marijuana. The proportions of those who used other illicit drugs prior to (or without any) marijuana use are for the most part less than 5 percent." Frequent marijuana use may be physically and emotionally harmful and is also associated with a host of other social and behavioral problems, including isolation, poor academic performance, violence, and crime. (Contains 2 tables and 7 online resources.)