It has been 20 years since EU citizenship was introduced under the Treaty of Maastricht, yet it remains a topical and contemporary issue. The main research question in this study is whether and how European citizenship affects the constitutionalization of the EU. It connects European citizenship with European constitutional processes, in order to grasp the constitutional relevance of EU citizenship. To this end, the different aspects of European citizenship are specifically linked to features that are commonly found in constitutions. The effect of and the relationship between four of these constitutional 'building blocks' and European citizenship are assessed. First, the effect of European citizenship on the vertical division of powers between the Member States and the EU is analyzed. Second, the way European citizenship relates to a common ideology, consisting of fundamental rights protection and democracy, is discussed. Third, the effect of European citizenship on judicial review and the hierarchy of norms in the EU is addressed. And finally, the question is posed as to how and to what extent European citizenship affects the constitutionalization of the EU and how European citizenship can be placed in the European constitutional context. (Series: European Administrative Law - Vol. 9) Subject: European Law, Constitutional Law, Administrative Law]