The relative abuse potential and dependence liability of the three major classes of opioids -- agonists, agonist-antagonists, and antagonists -- are reviewed. Opioid agonists include morphine, the prototypical narcotic analgesic, codeine, heroin, methadone, and propoxyphene. Pentazocine, buprenorphine, and nalbuphine are several of the agonist-antagonist analgesic drugs discussed here. Naloxone is an opioid antagonist which produces few effects unless administered subsequent to an opioid agonist. Opioid dependence is characterized by physiological modifications of the organism and results in physical addiction. Conditioned withdrawal is marked by pharmacological factors as well as behavioral processes. Detoxification, methadone maintenance, mixed agonist-antagonists, and non-opioid detoxification aids are included among the accepted treatment modalities for opioid dependence. Naltrexone is often used as a relapse prevention measure following detoxification. 19 references