The gateway hypothesis, which holds that cannabis use causes the use of other illicit drugs through some unspecified pathway, has been a controversial contention in research literature on drug abuse. This article explores the major competing explanations of the gateway hypothesis of drug abuse, beginning with a review of the relationships consistently found between the use of cannabis and the use of other types of illicit drugs. Three relationships are described between cannabis use and the use of other illicit drugs: (1) cannabis use precedes the use of other illicit drugs; (2) the earlier cannabis is used in the lifecycle, and (3) the more regularly it is used, the more likely it becomes that other illicit drug use will follow. The three main competing explanations of the relationship between cannabis and other illicit drug use are reviewed and include the explanation that the pharmacological affects on the brains of cannabis users increases the likelihood of using other illicit drugs. Recent research pertaining to the gateway hypothesis is reviewed, including discordant twin studies and longitudinal epidemiological studies. These research findings, taken as a whole, indicate that some of the association between cannabis use and other illicit drug use can be explained by the selective recruitment of heavy cannabis users with other pre-existing and aggravating traits as research subjects. The authors offer suggestions for research projects that would provide the necessary evidence to properly examine the gateway hypothesis. References