Acting under its Clean Air Act (CAA) authority, the U.S. Environmental Protection Agency (EPA) has attempted to regulate air quality on behalf of Native American tribes. However, the D.C. Circuit—in reviewing EPA’s tribal CAA rules—significantly cut back on these efforts, resulting in state encroachment on the environmental authority congressionally delegated to tribes. This undermines tribes’ sovereignty, control over their natural resources, and opportunities for economic development. EPA thus should change its approach in future CAA rules for Indian country, by relying on preemption, rather than its nebulous gap-filling authority in the statute.