For years now, courts and commentators have struggled to reconcile the presumption against preemption—the interpretive canon that presumes against federal incursion into areas of traditional state sovereignty—with the Court’s Chevron doctrine, which instructs courts to defer to reasonable agency interpretations of ambiguous federal statutes. Where Congress’s preemptive intent is ambiguous, should courts defer to agency interpretations under Chevron, or do preemption’s federalism implications demand a less deferential approach? Despite numerous opportunities, the Supreme Court has failed to clearly define the level of deference due to preemptive agency interpretations. In some cases the Court appears quite deferential and in others almost...