Drugs of abuse produce widespread effects on the structure and function of neurons throughout the brain's reward circuitry, and these changes are believed to underlie the long-lasting phenotypes that characterize addiction. Ecstasy (3,4-methylenedioxymethamphetamine) is a drug often in the headlines owing to concerns about its contribution to the death of young people and also to whether its widespread use will result in an ‘epidemic’ of people with cognitive problems later in life. To date, most studies have investigated the effects of Ecstasy on adult animals. Although loss of serotonergic neurons in non-human primates has had a powerful impact on how the danger of ecstasy is perceived, whether it occurs in humans remains controversial. This thesis focuses on microstructures of specific cognitive subcomponents as they may be more likely to reveal deficits than using standard batteries of complex executive tasks. More specifically, the research presented in this thesis uses cognitive functional magnetic resonance imaging (fMRI) in chronic ecstasy users to investigate differences in neural activity related to response inhibition, learning and memory functioning, processing of drug-related cues, and emotion regulation. Given that the literature is suggestive that females may be more susceptible to the chronic effects of ecstasy sex is included as an independent variable, and menstrual cycle phase and contraception usage is controlled for between ecstasy and control groups.Using fMRI this thesis has provided strong evidence for alterations in the executive components of cognitive and emotional neuronal processing in ecstasy users. Dysregulations were observed in ecstasy users with respect to response inhibition, learning and memory functioning, processing of drug-related cues, and emotion regulation. Furthermore, these results were independent of sex, promoting future studies to take menstrual cycle in to consideration and reducing the rationale for excluding women from pharmacological studies.