This chapter presents the theoretical device of autonomisation that the book uses throughout its analysis of Union citizenship. The chapter first looks into how some other researchers understand autonomous EU citizenship. Then it proceeds to construct its own tailor-made notion of autonomisation. To do so, the chapter first turns to the autonomy of EU law, one of its most notable defining features. It then looks into the notion of autonomous concepts. In the end, the chapter presents the theoretical lens of autonomisation for the subsequent analysis of the different dimensions and facets of Union citizenship carried out in this book.