Marcus Klamert’s book on ‘Th e Principle of Loyalty in EU Law’ is a welcome addition to this series. It is an important subject, and the book is divided into four parts. In the fi rst part, the author considers the way in which Article 4(3) TEU was drafted and its relevance for the themes in this book. We are introduced to what the author regards as specifi cations of loyalty, such as loyalty and confl ict resolution, and loyalty and duties of abstention. We are introduced also to the addressees of the loyalty obligation, with discussion of horizontal loyalty, vertical loyalty and reverse vertical loyalty, the latter connoting obligations fl owing from the EU to the Member States, rather than vice-versa. Klamert considers the relationship between loyalty and other doctrinal concepts such as good faith and pacta sunt servanda, as well as the federal dimension to fi delity, with comparative insights drawn from US, Canadian, and Australian law, and from a number of continental legal systems, such as Belgium, Austria and Germany. Th ere is analysis of the extent to which the concept of loyalty can be said to have informed the CJEU’s case law in seminal decisions such as Costa, ERTA and Francovich . In the second part consideration is given to the way in which loyalty fosters the cohesion of EU law. Klamert argues that loyalty informs, underpins or shapes legal concepts developed by the EU courts in order to ensure that EU law can function in a cohesive manner. Th is leads to examination of supremacy, pre-emption, and the principle of eff ectiveness, although Klamert recognizes the diversity of meaning accorded to the concept of eff ectiveness in the CJEU’s case law. Th e focus in part three of the book shifts to the role played by loyalty in relation to cooperation in EU law. Th is provides the setting for examination of the rules concerning delimitation of competence between the EU and the Member States, and the role played by loyalty in the interpretation and application of the respective fi elds in which the EU and the Member States can exercise power. Th ere is an overview of the distribution of competence in the post-Lisbon world, followed by analysis of the way in which loyalty impacts on the fi eld of non-exclusive competence. Th e discussion includes consideration of the way in which loyalty plays a role in relation to certain kinds of secondary EU legislation. Th e fi nal part of the book deals with loyalty and the construction of the EU. Th e objective is to draw together certain more general conceptual issues that have occupied the courts and academic commentary, and to reveal the ways in which loyalty plays a role in relation to their development and content. Th is includes exploration of cooperation/ confl ict in the EU, and analysis of the extent to which loyalty can be seen as an independent source of obligation and as a general principle of law. Th is is a thought-provoking book that will be of interest to all those who study EU law.