In a 6-0 decision, with the other three justices not participating, Justice John Paul Stevens said that if the meaning of a law like the Clean Air Act was not completely clear and a government agency such as the EPA offered a reasonable interpretation, federal courts must “defer” to the agency. That doctrine, known as “Chevron deference,” has since been cited in at least 19,000 cases, the most ever for any Supreme Court ruling on issues of administrative law.