The illicit drug 3,4-methylenedioxymethamphetamine (MDMA; Ecstasy) has been shown to cause long-term serotonin (5-HT) deficits in rodents and non-human primates, and based on the results of brain imaging studies in frequent Ecstasy users, it appears that MDMA has the potential to affect the human brain in much the same way. Because the 5-HT system is known to take part in the regulation of impulsivity, it has been hypothesized that increased impulsivity may be one of the consequences of MDMA exposure seen in Ecstasy users. The purpose of this review is to evaluate the evidence for a relationship between Ecstasy use and higher levels of impulsivity. Cross-sectional studies that used measures of impulsivity to compare abstinent Ecstasy users to non-users will be summarized, and longitudinal studies that attempted to find changes in impulsivity in Ecstasy users will be examined. In closing, the overall evidence for an association between Ecstasy use and higher levels of impulsivity will be evaluated, and potential explanations for this association will be discussed.