This chapter examines the development and operation of the European Union from a comparative politics perspective. It first considers the evolution of the EU, from the signing of the Treaty of Paris in 1951 that established the European Coal and Steel Community (ECSC) to the admission of Lithuania in 2015 as the nineteenth member of the Eurozone, and the UK’s decision to leave the EU in 2016. The chapter then explores the process of European integration and goes on to explain what it means to think of the EU as a political system. It also describes the two basic dimensions of the EU system: the vertical dimension (the EU as a ‘regulatory state’) and the horizontal dimension (the design and operation of EU decision-making). The chapter concludes by analysing the ‘missing link’ in the EU system— the lack of genuine democratic politics.