Signed by Richard Nixon on December 31, 1970, the 1970 Clean Air Act Amendments made federal air-quality regulation comprehensive: EPA sets National Ambient Air Quality Standards (NAAQS) for common pollutants, states devise State Implementation Plans, and stationary sources and vehicles must meet federal emission standards. Major amendments in 1977 and 1990 tightened requirements and added the acid-rain cap-and-trade program. The Supreme Court held in Massachusetts v. EPA (2007) that greenhouse gases fall within the Act's definition of 'air pollutant,' opening the door to federal climate regulation — later constrained by West Virginia v. EPA (2022).