CAA regulates pollution-generating emissions from both stationary sources, such as factories and power plants, and moving sources, such as cars, trucks, and aircraft. This case is about stationary sources. Title I charges D with formulating national ambient air quality standards (NAAQS) for air pollutants. D has issued NAAQS for six pollutants: sulfur dioxide, particulate matter, nitrogen dioxide, carbon monoxide, ozone, and lead. The Prevention of Significant Deterioration” (PSD) provisions make it unlawful to construct or modify a “major emitting facility” without a permit. A “major emitting facility” is a stationary source with the potential to emit 250 tons per year of “any air pollutant” (or 100 tons per year for certain types of sources). Facilities seeking to qualify for a PSD permit must comply with emissions limitations that reflect the “best available control technology” (BACT) for “each pollutant subject to regulation under” the Act. It is unlawful to operate any “major source,” wherever located, without a permit. A “major source” is a stationary source with the potential to emit 100 tons per year of “any air pollutant.” Although the statute sets numerical thresholds (100 or 250 tons per year) for emissions that will make a facility “major,” it does not specify by how much a physical or operational change must increase emissions to constitute a permit-requiring “modification.” Nor does it say how much of a given regulated pollutant a “major emitting facility” must emit before it is subject to BACT for that pollutant. D has established pollutant-specific numerical thresholds below which a facility’s emissions of a pollutant, and increases therein, are considered de minimis for those purposes. In Massachusetts v. EPA, this Court held that Title II of the Act “authorized D to regulate greenhouse gas emissions from new motor vehicles” if D “formed a ‘judgment’ that such emissions contribute to climate change.” In response to Massachusetts v. EPA, D promulgated greenhouse-gas emission standards for new motor vehicles and made stationary sources subject to the PSD program and Title V on the basis of their potential to emit greenhouse gases. D “tailored” the programs to accommodate greenhouse gases by providing that sources would not become newly subject to PSD or Title V permitting on the basis of their potential to emit greenhouse gases in amounts less than 100,000 tons per year. D realized that so many sources emitted 250 tons of greenhouse gases that it could not administer the program, hence the new 100,000-ton limit for greenhouse gases. Facilities already subject to permitting had to monitor greenhouse gases. Under D’s view, once greenhouse gases became regulated under any part of the Act, the PSD and Title V permitting requirements would apply to all stationary sources with the potential to emit greenhouse gases in excess of the statutory thresholds: 100 tons per year under Title V, and 100 or 250 tons per year under the PSD program depending on the type of source. D announced that beginning on the effective date of its greenhouse-gas standards for motor vehicles, stationary sources would be subject to the PSD program and Title V on the basis of their potential to emit greenhouse gases. P appealed. The appeals court ruled for D, and the Supreme Court granted certiorari.