Research has shown that adolescent users of tobacco are much more likely to progress to use of illicit drugs than are nonusers of tobacco. This article suggests potential psychosocial reasons for the progression based on principles of learning theory, Theory of Reasoned Action, Health Belief Model, and Cognitive Dissonance. In addition, a neuropharmacologic causal mechanism is discussed. The existence of tobacco's gateway function has important implications in the nation's efforts to reduce illicit drug use and adolescent smoking.