The purpose of this thesis is to examine the procedural democratic legitimacy of the Court of Justice of the European Union. The Court of Justice has been instrumental in the construction of the European Union. Through its interpretation of the Treaty of Rome since the 1960s, it has constituted a legal system distinctive in kind. In contrast to orthodox instances of the political community – international organisations and the nation-state – the EU exemplifies no general type. Its legal, constitutional, political, economic and social infrastructures are part of a complex and pervasive web of overlapping jurisdictions that goes some way beyond the ordinary international organisation (by virtue of constitutional principles such as direct effect and citizenship), but not quite as far as the nation-state (e.g. sovereignty contestation). This being the case, its interlocutors have long since understood that the EU is in a state of transformation – it is itself a project and a process, the end result of which (finalité) is unknown. As such, many questions have been asked about the legitimacy of this process; and, given the Court of Justice’s (in)famous generative role within this process, the Court also finds itself the subject of such scrutiny. The legitimacy of the Court of Justice has been the focus of attention from both academics and practitioners. Most of that attention has been on the Court’s jurisprudence and jurisdiction – scrutinising the legal reasoning of cases; or questioning the limits of its constitutional functions according to axiomatic conceptions of, for example, the separation of powers doctrine. By contrast, less attention has been paid to the democratic legitimacy of the Court of Justice, and much less in relation to the Court’s institutional design. The subject-matter of the analysis in this thesis is the Court’s structures and processes, such as: the composition and appointments processes for members of the Court; the mechanisms that give access to various kinds of participants (such as locus standi and third-party intervention); and the use of judicial chambers. Procedural democratic legitimacy, moreover, has two dimensions: intrinsic and instrumental. The intrinsic is a measure of the democratic credentials of the Court as a discrete decision-making authority (such as representativeness and democratic participation); whereas the instrumental is concerned with the ways in which the Court contributes to the overall democratic legitimacy of the EU. In this thesis, the structures and processes of the Court of Justice are examined in light of both of those criteria. In contrast to prevailing approaches of constitutional theorists – who tend to treat these criteria as functions that are quite discrete, and their performance as mutually exclusive – an important theoretical contribution of this thesis is to develop an analytical framework that allows for the inherent synergies and tensions that exist between intrinsic and instrumental criteria to be factored into analyses of the democratic legitimacy of constitutional courts.