Chapter 5 addresses the role of the foundational principles of EU law – primacy, direct effect, consistent interpretation and state liability – in ensuring the proper functioning of the internal market. The EU-specific principles are compared with their equivalents in public international law and in the EEA, the Energy Community, the ECAA and the Transport Community legal orders. The analysis helps to determine the implications of a possible absence of the supranational principles in the agreements exporting the acquis on the effectiveness of the acquis transferred to third countries by means of agreements operating under public international law.