How should political power be divided within and among national peoples? Can the nineteenth-century theory of the sovereign and unitary State be applied to the social reality of the word of the twenty-first century? If not, what constitutional and philosophical theories can make sense of the empirical and normative world of our times? The rise of international organizations and, within Europe: the emergence of the European Union, have challenged the idea of the sovereign state ever since the end of the Second World War. And, in a seemingly paradoxical parallel, the myth of monolithic state power has equally come under attack from within states. Indeed: many modern states currently experience a political and constitutional devolution (Italy, Spain, United Kingdom).Can we identify these developments with federal theory? Federalism is a philosophical and a constitutional problem. The federal principle has made a long ‘journey through time in quest of a meaning’ (SR Davis). Modern federalism emerged with the rise of the European State system. And in the nineteenth century, the federal idea became reduced to the idea of the Federal State. In the light of this reductionism, international centralisation and regional decentralisation continue to be – largely – misunderstood by mainstream European legal scholarship. The proposed project would fill this gap by providing a comparative constitutional analysis of the federal principle. It will analyse the historical evolution of the principle from the eighteenth to the twenty century in the first part, and concentrate on three contemporary experiences of “dividing” or “sharing” sovereignty in the second. The three case studies of (neo)federalism in the 21st century correspond to three legal sphere: the international sphere (United Nations, World Trade Organisation), the supranational sphere (European Union, NAFTA), and the regional sphere (Spain, United Kingdom).