In 1990, Congress passed amendments to the Clean Air Act, 42 U.S.C. §§ 7401 et seq., that granted Indian tribes the same power to regulate air quality on their reservation lands as states had on state lands. This included the power to create implementation plans aimed at ensuring that air quality met national standards, the ability to designate land to comply with certain standards, and the ability to create a permitting program for potential air polluters. Shortly afterward, the Environmental Protection Agency (EPA) (defendant) proposed rules implementing the amendments. The EPA determined that the amendments were a delegation of federal authority to the Indian Tribes that permitted the tribes to regulate all air quality within the boundaries of the reservation. The EPA further determined that reservation lands included lands held in trusts. These proposed rules were ultimately adopted as the final Tribal Authority Rule. Arizona Public Service Company (plaintiff) challenged the adoption of the Tribal Authority Rule on two grounds. First, it argued that Congress did not delegate to Indian tribes the authority to regulate air quality on lands held by nonmembers within a reservation. Second, it argued that Congress did not mean for the term “reservation” to include trust lands.