 This is a landmark decision, upending four decades of jurisprudence, a doctrine known as Chevron deference. It was named after the oil and gas company that had a key environmental case before the court 40 years ago. In overturning Chevron, Chief Justice John Roberts says courts should no longer defer to a federal agency's reading of an ambiguous statute. Conservatives have sought this outcome for years, and liberals now worry the ruling could affect thousands of cases in which federal regulators prevailed.