Abstract: The most famous case in administrative law, Chevron U.S.A. v. Natural Resources Defense Council, Inc., has come to be seen as a counter-Marbury, or even a McCulloch v. Maryland, for the administrative state. But in the last period, new debates have broken out over Chevron Step Zero - the initial inquiry into whether Chevron applies at all. These debates are the contemporary location of a longstanding dispute between Justice Scalia and Justice Breyer over whether Chevron is a revolutionary decision, establishing an across-the-board rule, or instead a mere synthesis of preexisting law, inviting a case-by-case inquiry into congressional instructions on the deference question. In the last decade, Justice Breyer's case-by-case view has enjoyed significant victories. Two trilogies of cases - one explicitly directed to the Step Zero question, another implicitly so directed - suggest that the Chevron framework may not apply (a) to agency decisions not preceded by formal procedures and (b) to agency decisions that involve large-scale questions about agency authority. Both of these trilogies threaten to unsettle the Chevron framework, and to do so in a way that produces unnecessary complexity for judicial review and damaging results for regulatory law. These problems can be reduced through two steps. First, courts should adopt a broader understanding of Chevron's scope. Second, courts should acknowledge that the argument for Chevron deference is strengthened, not weakened, when major questions of statutory structure are involved.