The rise of the European Union elicits both theoretical and practical questions about notions of citizenship, and citizens’ duties, that transcend nation-state boundaries. Indeed, its supranational nature invites reevaluation of the concepts of citizenship and political community more generally. In a similar vein, this chapter considers the European Court of Justice’s (ECJ) practice of horizontal application in light of republican theory. The fact that the ECJ has introduced horizontal application in EU law at all is itself a point of interest, given the debatable status of the Union as a political community in the republican sense. This book’s republican framework points toward a conceptual relationship between the development of horizontal application and the fate of the European Union as, in fact, something more than a loose union of nation-states. Put differently, a full flowering of horizontal application is theoretically tied to wider acceptance of the European Union as a fully fledged political community, complete with citizens’ duties to one another and a common good of which to speak.