Since the mid 1990s, MDMA has been increasingly used as a recreational drug called "Ecstasy" by young people in Europe and the United States. However, despite the widespread recreational use of Ecstasy, systematic data on the psychological and neurobiological effects of MDMA have been scant. To further our understanding of the mechanism of action of MDMA, the authors conducted several studies in healthy human volunteers in an effort to characterize the psychological, cognitive and behavioral effects of MDMA in healthy human volunteers. Prospective placebo-controlled within-subject study designs and standardized psychometric ratings and neuropsychological tests were used to assess the acute, short-term and prolonged effects of the drug. To elucidate the role of various neurotransmitter and receptor systems involved in the action of MDMA in humans, the blocking effects of specific receptor antagonists on MDMA-induced psychological alterations and measures of sensory information processing were studied. To identify the functional neuroanatomy involved in the action of MDMA, Positron Emission Tomography (PET) was used. The present contribution summarizes the acute effect of MDMA on psychological and cognitive measures, information processing, and regional brain activity in healthy human volunteers.