The book examines the phenomenon of dual nationality in the European Union, particularly against the background of the status of European citizenship - a status that is linked to the nationality of each EU Member State. While the first part sets out the approach towards (dual) nationality in Public and Private International Law as well as in EU Law, the second part consists of an overview of the dual nationality regimes in France, Italy, the Netherlands and Spain. The book shows that the autonomy of Member States in the field of nationality law is becoming increasingly problematic for the EU, and the author takes the position that there is arguably a need for the (minimum) harmonization of European nationality laws.