The 1990 amendments to the Clean Air Act (CAA) required each State to formulate, subject to approval by the Environmental Protection Agency (EPA) (defendant), a state implementation plan (SIP) designed to achieve national ambient air quality standards. An SIP could be challenged in federal court within 30 days of approval, or after 30 days if new information was learned by the petitioner. The State of Missouri submitted its plan to the EPA for approval of a reduction in sulfur dioxide emissions around the St. Louis metropolitan area. The EPA Administrator approved Missouri’s SIP. Thereafter, Union Electric Company (UEC) (plaintiff), St. Louis’ largest emitter of sulfur dioxide, challenged the State’s SIP in federal court after the 30 days period had lapsed. UEC argued that it could not comply with the State’s SIP because its sulfur dioxide-reducing emissions requirements were economically and technologically impossible. The court of appeals dismissed UEC’s petition for lack of jurisdiction. The U.S. Supreme Court granted certiorari to review.