Recent attention has focused on clean indoor air laws as part of a comprehensive tobacco control strategy. Clean air laws have the potential to reduce smoking behaviors as well as second-hand smoke. Many states do not have these laws but most firms now have policies restricting smoking indoors. The authors developed a framework intended to guide in the evaluation and improvement of clean air laws. This article examines the relationship between public laws and private restrictions and how smoking restrictions are likely to affect smoking behavior throughout the population. The article also discusses the information needed to better understand clean air laws and improve future public policy.