Through an analysis of the discovery of the doctrine of direct effect by the European Court of Justice, this paper argues that judicial interpretation of the EU constitutional treaties constitutes an underexplored aspect of EU supranationalism. The paper distinguishes between two standards of judicial interpretation: a teleological approach called for in the Vienna Convention on the Law of Treaties and applied by international courts and tribunals, and the doctrine of effet utile, which was developed by national courts in Europe. These approaches are not inconsistent with each other, but the latter represents a more muscular exercise of judicial interpretation that the ECJ has used to develop substantive areas of EU law. The paper draws on language from the ECJ rulings in Van Gend and Van Duyn to argue that the Court’s use of both teleology and the effet utile standard reflects a mixed approach to judicial interpretation that, at least in the context of direct effect, reflects the supranational nature of EU law.