Research on federalism is rarely concerned with its philosophical foundations. However, arguments on why and how best to organise a plurality of states in a multilevel political order have first been discussed by philosophers and continue to inspire contemporary reasoning on international and supranational relations not only in political philosophy. This book offers a unique overview of the philosophical foundations of federalism from both a historical and a systematic perspective. The analyses proposed by renowned scholars from the US and from several European countries cover classic writers such as Hobbes and the authors of the Federalist Papers, Kant and Rawls, and range from anthropological justifications of federal orders to contemporary problems of EU constitutionalism, the principle of subsidiarity and the jurisdiction of the European Court of Human Rights (ECHR). The book is of relevance to anyone interested in philosophical justifications of federalism.