Morphine (Fig. 1) remains the most powerful analgesic but its therapeutic value is compromised by its strong reinforcing properties, making it highly addictive. This has stimulated the quest to find an analgesic lacking dependence-producing properties. So far no such drug has been discovered. However, this search has helped to disclose the large range of endogenous opioid peptides and the various receptors on which they act. This diversity and opportunity for interaction between exogenous drug and endogenous peptide, makes opiate dependence, tolerance and withdrawal complicated phenomena that are not fully understood at the molecular and cellular levels. The occurrence of opiate receptors in 'reward' pathways also makes them possible targets for the treatment of other addictions, adding a further dimension to the picture.