What should be the most important topic for transnational sociology in the 21st century? Some cases at the end of the 20th century signify a challenge to the nation-state. Transnational sociology should propose a new citizenship model which can replace that of the nation-state. The challenge to the nation-state derives from the acceleration and diversification of international migration, emerging universal human rights and a fear of losing social integration. The alternative models, which emphasize citizenship, are incompatible with the nation-state model. Shared culture, presupposed by birth or blood, cannot legitimize a new citizenship model for the 21st century. Transnational sociological imagination is needed in order to explore whether domicile or other factors can form the basis on which a new citizenship model can be established.