The Environmental Protection Agency (EPA) (defendant) issued a notice of proposed rulemaking to (1) identify and classify nine characteristics typical of hazardous wastes and (2) provide a list of wastes definitively determined to be hazardous. The EPA’s proposed regulation would treat non-listed wastes as non-hazardous, as long as testing showed none of the nine identified characteristics. The listed hazardous wastes would be subject to regulation, but waste generators could elect to perform tests demonstrating that a presumptively hazardous waste should be exempt. During the comment period, industry participants raised generalized concerns about the proposed rules, including the proposed listing system. The EPA also indicated that it would provide a procedure for de-listing wastes improperly listed as presumptively hazardous. After notice and comment, the EPA issued two final rules that significantly changed the regulatory scheme initially proposed. First, the mixture rule provided that any mixture including a listed hazardous waste was presumptively hazardous. Second, the derived-from rule provided that any residue derived from the treatment of hazardous wastes was also hazardous. Shell Oil Company and other companies (plaintiffs) petitioned for review of the EPA’s final rules, arguing that the rules had been issued in violation of the procedural requirements for rulemaking under the Administrative Procedure Act (APA), 5 U.S.C. § 551 et seq.