The Natural Resources Defense Council, Inc. (NRDC) (plaintiff), a non-profit entity, filed a petition in federal court seeking review of an Environmental Protection Agency (EPA) (defendant) rule that allowed “fundamentally different factor” (FDF) variances to be issued to indirect dischargers of toxic pollutants in certain circumstances. The NRDC claimed EPA’s rule was promulgated in contravention to § 301(l) of the Clean Water Act (CWA), as amended in 1977, which provided that the EPA Administrator could “not modify any requirements” in the section as it pertained to toxic pollutants. Chemical Manufacturers Association and EPA argued that § 301(l) prohibited only those modifications mentioned in other provisions of the section. The court of appeals held for NRDC and found EPA’s rule was barred by the statute. The U.S. Supreme Court granted certiorari to review.