This chapter describes how the Clean Air Act (CAA) lays the foundation for US cooperative environmental federalism programs. The CAA was the first federal pollution law to significantly expand the federal government’s role beyond providing information and financial assistance to the states and regulating interstate pollution. At the same time, the Act preserved an important policy-making role for states choosing to participate. Despite its path-breaking federalism structure, the CAA’s approach is sometimes misunderstood, as reflected in stunning mischaracterizations of that structure in recent judicial decisions, including an important Supreme Court case. To set the record straight, the chapter explores why Congress embarked on this cooperative federalism venture, emphasizing the manner in which the CAA responds to collective action problems. It also evaluates the CAA’s implementation, identifying CAA cooperative federalism success stories that include the leadership provided by states such as California in regulating pollution from motor vehicles and, potentially, state and regional efforts to restrict greenhouse gas emissions that contribute to climate change. Its failures include the persistent inability of some states to achieve the national ambient air quality standards and, ironically given its early provenance, the program to control interstate pollution. The chapter concludes by assessing whether the CAA’s cooperative federalism model is a good fit for mitigating greenhouse gas emissions that contribute to climate disruption and suggesting alternative mechanisms for addressing intractable air pollution problems.