Section 109(a) of the Clean Air Act requires the Administrator of the EPA to promulgate National Ambient Air Quality Standards (NAAQS) for each air pollutant for which 'air quality criteria' have been issued under § 108, 42 U. S. C. § 7408. The Administrator must review the standard 'at five-year intervals' and make 'such revisions ... as may be appropriate.' American Trucking Associations, Inc., et al. (P)-challenged the new standards in the Court of Appeals for the District of Columbia Circuit, pursuant to 42 U. S. C. § 7607(b)(1). The District Court agreed that § 109(b)(1) delegated legislative power to the Administrator in contravention of the United States Constitution, Art. I, § 1 because it found that the EPA had interpreted the statute to provide no 'intelligible principle' to guide the agency's exercise of authority. The court thought that the EPA could perhaps avoid the unconstitutional delegation by adopting a restrictive construction of § 109(b)(1), so instead of declaring the section unconstitutional, the court remanded the NAAQS to the agency. Eventually, the Supreme Court granted certiorari.