The relative abuse liability is first evaluated through an examination of an epidemiological analysis of the young adult population. Four aspects of the addiction process are used to determine and compare dependence potential: the establishment and maintenance of drug-reinforced behavior, subjective effects, toxicity, and the extent to which tolerance and dependence are produced through patterns of self-administration. Several laboratory procedures are evolving to quantify the reinforcing potential within and between drug classes. Discriminative stimulus effects of PCP and hallucinogens have been analyzed in animal research and could be used to predict abuse liability in newly developed compounds. The behavioral toxicity of PCP has been quantitatively described through a wide range of analytic methods. Sensitive tests being developed to reveal withdrawal disturbances could be used to warn users when they are at risk for relapse long after drug use has stopped. 60 references