In a 6-2 decision authored by Justice Thomas, the Supreme Court held in consolidated cases Oklahoma v. EPA and PacifiCorp v. EPA that the Environmental Protection Agency (EPA)’s disapprovals of 21 individual state implementation plans (SIPs) under the Clean Air Act (CAA) are “locally or regionally applicable” actions, and, as such, must be challenged in their respective regional circuit courts—even when the EPA publishes them in a single, consolidated Federal Register notice.