The Defenders of Wildlife and others (plaintiffs) filed suit in federal court against National Association of Home Builders, the Environmental Protection Agency (EPA), the U.S. Fish and Wildlife Service (FWS), and others (defendants), seeking review of EPA’s decision to transfer National Pollutant Discharge Elimination System (NPDES) permitting authority to the State of Arizona pursuant to the Clean Water Act (CWA). Plaintiffs alleged that, pursuant to § 7(a)(2) of the Endangered Species Act (ESA), the EPA was required to consult with the FWS and receive a determination that no endangered or threatened species would be harmed by the transfer. The EPA disagreed and argued that § 402(b) of the CWA placed an unconditional mandate on the agency that it approve the transfer once all criteria had been satisfied. The court of appeals disagreed, invalidated the transfer, and held the EPA’s decision to be arbitrary and capricious. Defendants appealed. The U.S. Supreme Court granted certiorari to review.