The Clean Air Act directed the Environmental Protection Agency (EPA) (defendant) to consider promulgating regulations that would decrease hazardous air pollutants emitted by power plants. The act stated that the EPA should promulgate these regulations if the EPA found it “appropriate and necessary.” The EPA promulgated regulations decreasing emissions. The EPA declined to consider costs as a factor when determining whether to issue the regulations. The EPA later did consider cost in determining the extent of its regulation. The regulations cost power plants approximately $10 billion per year. The regulations were challenged in the United States Court of Appeals for the D.C. Circuit. The court of appeals upheld the regulations. The United States Supreme Court granted certiorari.