Justice Stevens’s 1984 opinion in Chevron v. NRDC may be completely unknown to most members of the public. But that opinion has had an enormous impact for the last thirty-five years in terms of how courts reason about their role in overseeing government. Indeed, in an interview while he was still on the bench, Justice Stevens himself referred to his approach to statutory interpretation, presumably including Chevron, as among his most significant judicial achievements. I think he was right. In Chevron, the key issue centered on a provision in the Clean Air Act authorizing the Environmental Protection Agency (EPA) to regulate sources of pollution. But the statute did not define what constitutes a “source.” EPA originally considered each and ...