Behavioral pharmacologists have assumed that the properties of drugs that mediate their discriminative stimulus effects are related to aspects of drug actions that result in their subjective effects in humans. The basis of this assumption is examined in this chapter. Evidence to support this assumption includes the formal properties of the learning process involved in acquiring both behaviors. Although the procedures used to train animals to learn a drug discrimination are explicit, an analysis of how humans learn to attach verbal responses to unobservable internal subjective states appears to involve a similar learning paradigm. Additional evidence of the commonality of the two effects is that the results from drug discrimination studies in animals and studies evaluating subjective effects in humans yield similar drug classifications. However, when subjective drug effects are analyzed in more detail, it is clear that the concordance between the two approaches is not always good. On the other hand, when drug discrimination and subjective effects are both measured in humans, an examination of the results generated when individuals respond differently to the same drug indicates that the hypothesis that their discrimination is based upon a profile of subjective effects is supported.