Empirical studies have found that policies aimed at reducing youth access to tobacco have been successful at increasing retail compliance, but their effects on actual tobacco use are mixed. This article presents a model of youth access policies that helps explain the apparently conflicting results in the extant literature, provides a framework for future empirical studies, and suggests implications for public policy. Our model highlights the interaction of components in a well-designed policy, including sufficient compliance checks, penalties, and community involvement. It also illustrates why it will be difficult to eliminate all of youth supply. Nonretail sources, such as borrowing or stealing from parents and siblings and purchasing from older peers through black markets, are an important component of youth supply and become more important as retail access is reduced. The analysis is limited to policies that affect the retail supply of cigarettes but suggests the need for other policies that affect the demand for cigarettes, such as taxes and cessation policies, in order to further reduce youth smoking rates.