This chapter begins the analysis of Union citizenship carried out in this book by giving a conceptual overview of Union citizenship. It deals first with the proper naming of the concept. Then this chapter proceeds to investigate the form of EU citizenship and engages with its classification. After this, the chapter outlines the research framework. Particularly, it focuses on the legal sources that regulate Union citizenship and looks into its definition, mission, quality of additionality, substance and scope. This chapter also locates the place of Union citizenship on the terminological scale of citizenship and nationality. Finally, after outlining some structural perspectives on Union citizenship, this chapter critically engages with the approach to EU citizenship as an essentially contested concept.