In 1946, Congress enacted the Atomic Energy Act (AEA), 42 U.S.C. § 2011 et seq, to regulate the production of nuclear energy. The Atomic Energy Commission (AEC) administered the AEA by, among other things, creating standards for the discharge of radioactive materials into waterways. Congress later enacted the Federal Water Pollution Control Act (FWPCA), 33 U.S.C. § 1251 et seq, to regulate water pollution, including pollution from radioactive materials. The Environmental Protection Agency (EPA) administered the FWPCA through a permit program. Only permit holders were legally permitted to discharge pollutants into waterways. EPA Administrator Russell Train (defendant) excluded from the permit program any radioactive materials already reached by the AEA’s regulatory scheme. The Colorado Public Interest Research Group, Inc., together with other environmental groups and citizens (plaintiffs), brought suit against Train, seeking an injunction directing the EPA to include in its permit program all radioactive materials discharged into waterways. The district court held that the AEC had exclusive authority to regulate radioactive materials covered by the AEA. The court of appeals reversed in favor of the plaintiffs, finding that the text of the FWPCA defined “pollutant” as including radioactive materials and provided no exceptions. The United States Supreme Court granted certiorari.