Environmental law practitioners are keenly aware of the deferential standard of review that applies when federal courts consider an administrative agency’s interpretation of a statute it is charged with implementing. As Chevron, 467 U.S. 837 (1984), instructs, if a statute is ambiguous or silent on a particular issue, thereby leaving some implicit “gap” to be filled, courts are bound to approve an agency’s reasonable determination on that issue. The core separation of powers principle underlying this deferential posture is the agency’s presumed institutional expertise in implementing legislation within the bounds of its broad policymaking authority. Applying the Chevron formula remains enigmatic, however, in large part because of its prescribed threshold search for statutory ambiguity. This problem likely was rendered more acute by the Supreme Court’s pronouncement in National Cable & Telecommunications Ass’n v. Brand X Internet Services, 545 U.S. 967, 982 (2005) that “[a] court’s prior judicial construction of a statute trumps an agency construction otherwise entitled to Chevron deference only if the prior court decision holds that its construction follows from the unambiguous terms of the statute and thus leaves no room for agency discretion.” The Court’s recent fractured decision in a tax case, United States v. Home Concrete & Supply, LLC, 132 S. Ct. 1836 (2012), underscores the vagaries of deciphering statutory ambiguity within the Chevron/Brand X paradigm.