Over the past decade, the relationship between European and international law has largely been commented through the prism of the autonomy, recent decisions issued by the Court of Justice of the European Union having emphasized the integrity of the EU legal system. Yet by committing the EU to contribute “to the strict observance and the development of international law”, the Lisbon Treaty provides for a slightly different approach in primary law. Article 3 (5) of the Treaty on European Union (TEU) traduces a deferent attitude that can be designated as one of loyalty towards international law. Beyond a general objective of the EU, the notion of loyalty may be viewed as a concept, explaining that EU law and international law interact in a subtle equilibrium between loyalty and autonomy. In addition to a conceptual interest, loyalty could be acknowledged as a metaconstitutional principle of EU law that would allow reconsidering the effects of international law in the European legal system.