The 1977 Amendments to the Clean Air Act (CAA) required polluters in certain areas to obtain a permit from a state regulator before building any new or modified stationary sources of air pollution. The state regulator could only grant the permit if the polluter met specific requirements regarding the abatement of new pollution. The Environmental Protection Agency (EPA) promulgated a rule interpreting the term “stationary source” to include what the agency called a “bubble policy.” Under this policy, an existing plant containing several pollution-emitting devices could install or modify one piece of equipment without a permit if the alteration did not increase the total emissions from the plant. The Natural Resources Defense Council (NRDC) (plaintiff) challenged the EPA’s interpretation of the word “source.” Specifically, the NRDC argued that the word referred to each individual pollution-emitting piece of equipment, which meant that a plant would need to obtain a permit any time it created a new source of pollution or modified an existing source if the effect were to increase the pollution from the source. Finding that this interpretation best served the goals of the CAA, the court of appeals agreed with the NRDC. In reaching this decision, the court recognized that Congress had not expressed an intent regarding the applicability of the bubble concept to the permit program. The United States Supreme Court granted certiorari to review the appellate court’s decision.