This chapter is about addiction, a problem that is perhaps the greatest challenge to our confidence in human rationality. The problem of addiction is not confined to a few seductive molecules. Addictive preferences are woven deeply into the fabric of civilized life, including those for normal substances (food, chocolate), structured activities that do not require a substance (gambling, day trading), emotional patterns (thrill seeking, destructive personal relationships), and the most elementary and pervasive form of regretted choice, procrastination. There have been many attempts to formulate a technical definition of addiction, but the results never coincide exactly with ordinary usage. For instance, the emergence of physiological signs of withdrawal has been a favorite of authors who want to restrict "addiction" to the realm of substance use, but discontinuing even heavy use of cocaine does not lead to physiological withdrawal, whereas stopping intense gambling activity sometimes does. We do not have to settle on the proper definition. The challenge to rationality is the same whether someone has an addiction or just a bad habit, as long as the person herself perceives the habit to be both bad and hard to break.