Opioids have long been known to inhibit sexual behavior. However, it is only within the last decade that the effects of opioids on sexual behavior have been studied extensively and a number of hormonal and neurochemical correlates established. In this review, the experimental literature on opioids and sexual behavior in humans and laboratory animals is examined. Clinical and anecdotal accounts of opioid use are also discussed, in addition to the pharmacology, neuroendocrinology, and biochemistry of opioid administration, to provide a synthesis of critical information. New research directions involving the study of endogenous opioid systems, opioid receptor subtypes, and the opioid modulation of neurotransmitter systems are outlined. Finally, a comprehensive bibliography of the human and animal literature is included.