Introduction; Part I: Introducing Loyalty; 1 Loyalty in the EU Treaties; 2 Loyalty in Context; 3 Loyalty and the Constitutionalization of EU Law; Part II: The Cohesion of European Union Law; 4 A Primer on the Structure of Union Law; 5 Supremacy, Pre-Emption, and the Union Interest; 6 Effectiveness, Judicial Protection, and Loyalty; Part III: Cooperation in the European Union; 7 A Primer on Union Competences; 8 Loyalty and Non-Exclusive Competences; 9 Manifestations of Loyalty in Secondary Law; 10 Loyalty and Mixed Agreements; Part IV: The Construction of the European Union; 11 A Primer on Constitutional Cooperation and Conflict in the EU; 12 On the Nature of Loyalty; 13 Deconstructing Loyalty; 14 Amplification

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Despite its seemingly innocuous wording, in what is now Article 4 (3) TEU, the principle of loyalty has had a significant impact in deepening the reach of EU law within the Member States. The duty of sincere cooperation has been interpreted strongly by the European Courts as imposing serious duties on States to give strong effect to European legal acts. The principle has been central to the development of Union law since the 1960s, and is still being relied on by the European Court of Justice
to often-controversial effect.
Providing a thorough discussion of the principle of loyalty in EU law, this book introduces a novel classification of the very diverse roles loyalty plays in the EU. It distinguishes between the effects loyalty prescribes for interlocking the legal orders of the Member States with Union law, its application in preventing and resolving conflicts between the Union and the Member States, and the loyalty principle's role in the shaping of EU law. It addresses important and yet unresolved
questions pertaining to loyalty, such as its relation to the principles of solidarity, pre-emption, the Union interest, institutional balance, and the unity of international representation. The book explains why loyalty has been neglected in the prevailing narratives about the foundational case law of the
European Court of Justice, and highlights its central importance to understanding EU public law.

Marcus Klamert is a legal officer with the Executive Office for Constitutional Matters of the Austrian Federal Chancellery and a lecturer at the Vienna University of Economics and Business. He was previously a Legal Officer with the European Commission (DG SANCO), Senior Lecturer at the Institute for European and International Law at the Vienna University of Economics and Business (WU), Fellow of the Austrian Academy of Sciences (APART), Visiting Fellow at the Institute of European and Comparative Law (IECL) at the University of Oxford, Visiting Fellow at the Law Department of the European University Institute (EUI), and has practised as an Associate with Freshfields Bruckhaus Deringer in Vienna. He holds a PhD from the University of Vienna, and a Masters from the University of Amsterdam. He has published on external relations, the effect of directives, European constitutional law, and the fundamental freedoms.