The holder of a federal NPDES permit is subject to enforcement action by the Administrator for failure to comply with the conditions of the permit. The Administrator's enforcement arsenal includes administrative, civil, and criminal sanctions. In the absence of federal or state enforcement, private citizens may commence civil actions against any person 'alleged to be in violation of' the conditions of either a federal or state NPDES permit. The Commonwealth of Virginia established a federally approved state NPDES program administered by the Virginia State Water Control Board (Board). In 1974, the Board issued an NPDES permit to D authorizing the discharge of seven pollutants from the company's meatpacking plant on the Pagan River. Between 1981 and 1984, D repeatedly violated the conditions of the permit by exceeding effluent limitations on five of the seven pollutants covered. D installed new equipment to improve its chlorination system in March 1982, and its last reported chlorine violation occurred in October 1982. The new chlorination system also helped to control the discharge of fecal coliform, and the last recorded fecal coliform violation occurred in February 1984. D installed an upgraded wastewater treatment system in October 1983, and its last reported TKN violation occurred on May 15, 1984. Ps sent notice in February 1984 to everyone indicating its intention to commence a citizen suit under the Act based on D's violations of its permit conditions. P filed this suit in June 1984, alleging that D 'has violated . . . [and] will continue to violate its NPDES permit.' The District Court granted partial summary judgment for Ps in August 1984, declaring Gwaltney 'to have violated and to be in violation' of the Act. D moved in May 1985 for dismissal of the action for want of subject-matter jurisdiction under the Act. Gwaltney argued that the language of § 505(a), which permits private citizens to bring suit against any person 'alleged to be in violation' of the Act, requires that a defendant be violating the Act at the time of suit. D contended that because its last recorded violation occurred several weeks before Ps filed their complaint, the District Court lacked subject-matter jurisdiction. The District Court held that § 505 authorizes citizens to bring enforcement actions on the basis of wholly past violations. The court of appeals affirmed holding that § 505 'can be read to comprehend unlawful conduct that occurred only prior to the filing of a lawsuit as well as unlawful conduct that continues into the present.' The Supreme Court granted certiorari.