The history of the European Union has been fraught with constant friction between the sovereignty of the Member States and the supranational powers of the Union, with the Union gaining terrain in fields of law traditionally belonging to the Member States. Despite this tension, certain legal fields are steadfastly asserted as belonging to the Member States. Notably, Member States regulate the grounds of the acquisition and loss of nationality. The Treaty of Lisbon highlights that the nationality of Member States is scarcely governed by European Union law, if at all. The sole provision governing the relationship between Member State nationality and Union law, i.e., Article 20 of the Treaty on the Functioning of the European Union (TFEU) stresses the primacy of Member State nationality.Reality, however, is often not as simple as such a cursory reading implies. European Union citizenship, once a mere complementary facet of the national citizenships, has transformed into an institution in its own right, forming a symbiotic relationship between the Member State nationality and the European Union.