Rationality is here considered from a functional viewpoint: How may the concept of rationality be best used in talking about addictive behavior? The article considers rationality in terms of overt behavioral patterns rather than as a smoothly operating logic mechanism in the head. The economic notion of rationality as consistency in choice - the property of exponential time discount functions - is examined and rejected. Addicts are not irrational because of the type of time discount function that governs their choices-or even because of the steepness of that function. Instead, rationality is here conceived as a pattern of predicting your own future behavior and acting upon those predictions to maximize reinforcement in the long run. Addicts are irrational to the extent that they fail to make such predictions and to take such actions.