From a global historical viewpoint, federal systems are the exception rather than the rule, even though examples of federalism can already be found in Greek antiquity. From the 17th century onwards there was a revival of federalism, but in the post-World War II-era of decolonization federal systems did not emerge as the most popular form of government. Still, there is considerable variation among federal systems, i.e. those political systems where governmental authority is constitutionally divided among a central «federal» government and «state» governments in such a way, that each type of government has final decision-making authority in at least a few policy fields; a federation may be either effective or formal, unitary or decentralized, symmetrical or asymmetrical. In order to explain the extent and the kind of federalism in specific cases, the formallegal structure, the socio-economic environment and the political behaviour of elites as well as citizens should be taken into account. Political behaviour should be taken as the key factor, without disregard, however, for its relationship to social factors (e.g. pluralism) and format structures (e.g. constitutions). In order to assess their relative importance, comparative research is required; Western Europe and the Third World (South Asia, Sub-Saharan Africa) offer highly promising prospects for this type of research. On the research agenda of federalism, special attention should be given to topics like the informal federal arrangements in cabinet formation, policy-making and public administration, federal party polities, and the impact of public policies on the federal socio-economic environment.