Smoking cessation is the principal means by which a current cigarette smoker can alter his or her future risk of disease. Prevention of smoking initiation among adolescents can reduce smoking prevalence, but adolescents contribute little to rates of smoking-related illness until they have been smoking for 30 or more years. Cessation is often examined at the individual level in order to deter-mine the effects of cessation interventions or to define individual predictors of who will or will not be successful in their cessation attempts. However, for these individual effects to create a substantive public health benefit, they must sum to create a significant change at the population level. Powerful interventions that affect only a few individuals will have little impact on disease rates, whereas weaker interventions that impact large numbers of smokers will have important and cumulative effects on disease rates. In addition, many interventions (e.g., price increases, changes in social norms, etc.) are delivered to the population as a whole rather than to individual smokers one at a time, and it is these population-based interventions that have formed the core of the tobacco control efforts currently underway in California, Massachusetts, and several other states. This volume examines cessation at the population level. By population level, we mean that all segments of society form the denominator for evaluation of the effectiveness of tobacco control interventions. Therefore, this volume relies heavily on representative surveys of smoking behaviors in state and national populations. By doing so, it defines measures of cessation that can be used to assess the effects of tobacco control programs or public policy changes on smoking behavior. It then uses those measures to identify who is quitting, who is being successful, who is being exposed to various tobacco control interventions, and which tobacco control interventions are proving effective.