© 2014 Elsevier Inc. All rights reserved. Why five decades since smoking's harms were first widely publicized do 20% of US adults smoke? Theoretical frameworks to explain smoking come in three forms: rational, imperfectly rational, and irrational addiction. A reason rational people smoke and others do not is preferences differ, for example, smokers attribute less value to being healthy. Persons believing they have low life expectancy or underestimate the probability of adverse consequences may be more likely to smoke. Although theoretical disputes remain, there is some consensus, including: Raising price reduces demand; demand falls with rising educational attainment; smokers earn less; and smoking adversely affects health. Public policies seek to reduce smoking and its negative externalities with limited success.