Friends of the Earth (FOTE) and several other environmental groups (plaintiffs) brought suit against Laidlaw Environmental Services (defendant) alleging that the company was violating provisions of the Clean Water Act (CWA) and its National Pollutant Discharge Elimination System (NPDES) permit by emitting mercury into the environment. Laidlaw claimed that the controversy was moot because it complied with all environmental regulations associated with its NPDES permit. The district court determined that, because Laidlaw was in compliance with its permit, it was inappropriate for FOTE to sue under the CWA. After this decision, Laidlaw closed one of its main offending facilities. On appeal by FOTE, the court of appeals held that the case was moot because Laidlaw’s compliance with the statute and closure of its offending facility constituted a voluntary cessation of the offending behavior. The United States Supreme Court then granted certiorari to determine the law surrounding mootness.