Vermont residents Ouellette and others (plaintiffs) filed a class-action nuisance lawsuit in Vermont state court against the International Paper Company (IPC) (defendant) alleging that the pollutants IPC discharged from its New York-based pulp and paper plant on Lake Champlain, and which traveled underwater via diffusion pipe to the banks of Vermont, diminished the value of nearby Vermont residents’ homes due to the unhealthy, smelly discharge in the water. Plaintiffs sought an injunction to stop the discharge as well as monetary damages. IPC removed the lawsuit to federal district court and, there, filed a motion for summary judgment claiming plaintiffs’ state law claims were preempted by the federal Clean Water Act (CWA), 33 U.S.C. § 1251, et. seq. The district court denied IPC summary judgment and held that a state action to redress interstate water pollution could be maintained under the state law in which the injury occurred. IPC appealed. The court of appeals affirmed. The U.S. Supreme Court granted certiorari to review.