The Environmental Defense Fund (EDF) (plaintiff) filed suit in federal district court against the City of Chicago, its mayor, and others (defendants), alleging that its method of incinerating solid waste produced ash that was toxic enough to qualify as a “hazardous waste” under § 3000(i), Subtitle C, of the Resource Conservation and Recovery Act of 1976 (RCRA) and subsequent regulations promulgated by the Environmental Protection Agency (EPA). Defendants argued the ash resulting from the incineration process was excluded from the requirements. The district court agreed and granted defendants’ motion for summary judgment. The court of appeals reversed. The U.S. Supreme Court granted certiorari to review.