Opioid receptors mediate the strong analgesic and addictive actions of exogenous opiates, the prototype of which is morphine. The opioid system consists of a family of endogenous opioid peptides and three receptor types, m, d and k. It is widely distributed throughout the CNS and regulates a large diversity of physiological functions, including pain perception and mood control. The recent cloning of opioid receptors has opened up a new era in opioid research. The molecular basis of opioid action may now be addressed by in vitro structure-function studies of recombinant receptors and by in vivo gene targeting. Novel drug design strategies based on data obtained from molecular approaches will be developed in order to generate the long-sought non-addictive analgesic.