The book examines the evolution and expansion of NATO and the EU, focusing on overlapping membership and competences.Since the creation of NATO and the EU, their memberships and territorial overlap have expanded significantly, while the EU's competences have grown beyond economic cooperation. As a result, dual NATO/EU Member States increasingly face potential conflicting obligations. This publication analyzes how such overlaps function under international law and how related conflicts can be interpreted and resolved.The book examines the evolution and expansion of NATO and the EU, focusing on overlapping membership and competences. It analyzes their legal frameworks and conflict resolution mechanisms, highlighting key treaty provisions and comparing the role of the Court of Justice of the European Union (CJEU) vis-à-vis NATO's political decision-making structures. Using examples from military mobility, procurement, data protection, and budgeting, it also addresses recent developments such as Finland and Sweden's accession to NATO. Combining conceptual analysis, comparative perspectives, and case studies, this research is a valuable resource for scholars and advanced students of public international law, EU external relations, and transatlantic governance. It also serves as a practical guide for legal practitioners and policymakers, helping them identify and address conflicts arising from overlapping NATO and EU treaty obligations at an early stage.J. (Jarin) Nijhof, LL.M., Ph.D., is Host Nation Attorney-Advisor of the United States Armed Forces in the Netherlands. He conducted his doctoral research in a personal capacity as an external PhD-candidate at the Faculty of Law of Maastricht University.