The main objective of this study is to examine the phenomenon of dual nationality in the European Union (EU), particularly against the background of the status of European citizenship - a status that is linked to the nationality of each EU Member State (Article 20(1) of the Treaty on the Functioning of the European Union provides that ‘citizenship of the Union shall be additional to and not replace national citizenship’). The study consists of two parts. The first part (Chapters 1 and 2) sets out the approach towards (dual) nationality in Private International Law and EU Law, in particular by analyzing the case law of the European Court of Justice (ECJ). The second part (Chapters 3- 6) consists of an overview of the dual nationality regimes in four EU Member States - France, Italy, the Netherlands and Spain -, and their possible effects on the EU as a whole. Chapter 2 of the thesis is entitled the ‘intra-EU context’, since it primarily deals with the ECJ’s approach towards a dual nationality consisting of two Member State nationalities. The country reports, on the other hand, deal with the ‘extra-EU context’ because the dual nationality policies of the countries under consideration predominantly affect non-Member State nationals. Thus, France and the Netherlands have for some time already faced the question how to integrate the (Muslim) immigrant population; Italy and Spain have long since adopted a system of preferential treatment for (Latin American) former emigrants and their descendants. The country reports demonstrate how dual nationality is used (or rejected) in these four countries. Finally, the question whether the EU should in time acquire (limited) competence in the field of European nationality law is one of the major themes of this study. Regardless of one’s stance on this question, it must be readily admitted that the subject of Member State autonomy in nationality law is becoming ever more salient with the enlargement of the Union and the growing relevance of European citizenship in the case law of the ECJ. In the opinion of this author, the study shows that the almost absolute autonomy of Member States in the field of nationality law is becoming increasingly problematic for the EU as a whole. Based inter alia on the findings from the country reports, this thesis takes the position that there is arguably a need for the (minimum) harmonization of European nationality laws.