It has been 20 years since Union citizenship was introduced under the Treaty of Maastricht, yet it remains a topical and contemporary issue. The year 2013, in which this study was finalised, has been designated the European Year of Citizens, “to enhance awareness and knowledge of the rights and responsibilities attached to Union citizenship.”2 In the past two decades, European citizenship and its impact have been the topic of debate in rather voluminous academic literature.3 These contributions focussed on subjects such as political rights, social rights, the scope of these rights and the interaction with national policy areas and more recently the constitutional consequences of European citizenship. In this volume the constitutional impact of European citizenship is analysed in a horizontal way, to explore the role of EU citizenship in the constitutionalisation of the European Union. To this end, the different aspects of European citizenship are specifically linked within a constitutional context. In this study it will be assessed whether the introduction of, and the case law and legislation on European citizenship have affected the system and nature of the European Union. Ever since its introduction in the Treaty of Maastricht, the concept of European citizenship has been criticised from different angles, either as being too weak, or as being too intrusive. Weiler stated in 1998 that “the Citizenship clause in the TEU is little more than a cynical exercise in public relations.”4 Others have stressed that European citizenship case law of the Court of Justice of the European Union (the Court or Court of Justice), after 1998, exceeded the limits of Union law.5 In national media, and in policies and documents of European institutions, Union citizenship has been a topical theme. In the case law of the Court, many Member States intervene in cases concerning Union citizenship. The initial fear, at its introduction, that Union citizenship would only be of symbolic value, that Union citizenship would be nothing more than pie in the sky, has proved unfounded. In contrast, European citizenship case law has, today, even been criticised as being too intrusive in national policies.