The scholarly literature on U.S. environmental federalism addresses a number of interesting and complicated policy and political questions. As the authors of the articles included in this virtual issue of Publius illustrate, federalism shapes the policy response to traditional problems such as air pollution, water pollution, and drinking water, in addition to issues such as climate change. As the United States continues to grapple with difficult environmental challenges, understanding the role of federalism will continue to be important, and an area ripe for additional theoretical and empirical inquiry.