10100 Chevron U.S.A. Inc. v. Natural Resources Defense Council, Inc. 467 U.S. 837 (1984), a landmark case in which the United States Supreme Court has put the legal test to determine whether to grant acquiescence to a government agency's interpretation of the law it administers. The resolution clarified a principle now known as "respect for Chevron". The principle consists of a two-part test applied by the Court, where appropriate, which is highly respected for government agencies: "Whether the agency's response is based on a permitted building [added emphasis] to the statute", as long as Congress does not speak directly about the exact issue in question. The decision included a lawsuit challenging the U.S. government's interpretation of the word "source" in an environmental law. In 1977, the U.S. Congress passed a bill amending the Clean Air Act of 1963 -- the United States Comprehensive Act regulating air pollution. The bill changed the law so that all companies in the United States that intend to build or install any major source of air pollutants must go through a "detailed review of the new source" before they can move forward. The bill did not specify exactly what constituted a "source" of air pollutants, so the EPA defined it as part of the implementation of changes to the law. The EPA's initial definition of "source" of air pollutants essentially included any change or significant addition to a factory or plant, but in 1981 changed its definition to a mere plant or plant as a whole. This has allowed companies to avoid the "new sources review" process entirely if they simultaneously increase their plant emissions by Construction or modification, by modifying other parts of its plant to reduce emissions so that the overall change in plant emissions is zero. The Natural Resources Defense Council, an American non-profit environmental advocacy organization, then filed a lawsuit challenging the legality of the new EPA definition. Chevron is one of the most important decisions in U.S. administrative law, and has been mentioned in thousands of cases since its promulgation in 1984.