A large body of evidence indicates that the endogenous opioid system plays an important role in maintaining alcohol drinking behavior. Research is reviewed indicating that the reinforcing properties of alcohol which lead to continued and repeated bouts of drinking are due, in part, to alcohol-induced activation of the endogenous opioid system. Opioid receptor antagonists decrease alcohol craving, alcohol consumption, and loss of control over drinking. The potential of opioid receptor antagonists to improve treatment outcome in comprehensive relapse prevention programs is discussed.