How much deference should courts afford executive branch interpretations of statutes and treaties? The question that has long engaged foreign relations scholars has found new salience as it has become apparent in recent years that the Supreme Court will neither abstain nor reliably defer to presidential judgment even in cases implicating national security. As the courts grapple with the scope of detention authority granted by Congress’ 2001 Authorization for the Use of Military Force, or the limits on that authority under the Geneva Conventions, a number of scholars have embraced administrative law deference doctrines such as that in Chevron v. Natural Resources Defense Council as a way of constraining newly vigorous judicial engagement with executive law interpretation. The courts may have some appropriate role in interpreting foreign relations law, the notion is, but Chevron at least requires that the courts accept any reasonable executive interpretation. Yet, as this Article contends, Chevron’s promise for foreign relations law elusive. The Chevron doctrine in administrative law has proven neither as stable nor as attuned to the executive’s functional strengths and weaknesses as its foreign relations advocates imagine. More, Chevron poses a still unresolved challenge to formal understandings of the judicial power under Article III of the Constitution. If the executive enjoys substantial authority in statutory and treaty interpretation, what remains at the core of the courts’ structural power to say what the law is? While administrative law scholars have long struggled with the question, it has received surprisingly scant attention in the contemporary foreign relations debate. This an oversight. Understanding what power – and duty – the courts have to say what the law is a necessary precursor to determining what interpretive relationship between the courts and the executive should come after the Court has rejected abstention or total deference. This Article introduces the formal dimension into questions of interpretation in foreign relations law. Concluding that predominant understandings of the judicial power of interpretation provide insufficient accounts of the Court’s role in this realm, it begins the task of developing a supplemental understanding of formal judicial power for foreign relations law.