This chapter examines the relationship between federalism and European integration, with the goal of demonstrating the relevance of the federal idea to the building of Europe. It first clarifies some fundamental concepts such as federalism, federation, confederation, the modern state, and European integration. It then considers federalist theory and practice, along with three strands of federalism that emerged after the Second World War: Jean Monnet’s ‘federalism by instalments’, Altiero Spinelli’s ‘democratic radicalism’, and ‘integral’, ‘personalist’, or Proudhonian federalism. The chapter proceeds by exploring comparative federalism and the insights it provides on the future shape of the European Union, the implications of liberal intergovernmentalism for federalism, and the ramifications of the evolution of the EU Constitution for federalism. Finally, it assesses the questions that the challenge of EU enlargement poses for federalism and the federalists.