The aim of this thesis is to analyse the extent to which criminal law can contribute towards our understanding of Union citizenship and of the political community of the Union. In carrying out this task it adopts a particular perspective on both criminal law and Union citizenship. Firstly, it adopts the criminal law theory developed by RA Duff, premised on the notions of citizenship and community; crimes are viewed as public wrongs, committed against the community. Individuals are held responsible as citizens and are called to account before the community. Secondly, it adopts a particular account of Union citizenship based on a distinction between transnational dimensions and supranational dimensions. The transnational dimension is then broken into two sub-dimensions based on the concepts of social integration and autonomy or a space of free movement. The role of criminal law in these dimensions of Union Citizenship is analysed in the main body of the thesis. Two chapters consider the role of criminal law in social integration in the context of the acquisition of residence rights and the serving of sentences. Two chapters consider the parallels between the autonomy of Union citizens that results in a single space of movement, and the area of justice as it is constructed through the European Arrest Warrant and the operation of a transnational ne bis in idem principle. A final substantive chapter details the competence of the Union to adopt legislation criminalising certain conduct and the extent to which this can be said to contribute to the formation of a community at a supranational level. A conclusion brings together the findings of the thesis in relation to Union citizenship and considers the implications for the structure of the political community in the Union. It is suggested the national remains the main site for communities in the Union. However, transnational processes associated with Union citizenship trigger the emergence of certain supranational norms and ultimately a composite, complementary supranational community.