While migration has always existed, and its consequences have always been important, few people have lived a mobile life in the history of mankind. Population immobility has recurrently been part and parcel of political strategies of social control and domination. Since the second half of the 20th century, however, the extent of geographical movements of individuals has expanded enormously. In particular, the size and scope of international travel has increased at an exponential pace. Favored by globalization and technological progress, transnationalism, initially linked to migration, has emerged as a relatively widespread phenomenon that involves a growing portion of the general population, especially, but not only, in developed countries. Mainly on the basis of research carried out in Europe, there is evidence that transnational practices tend to strengthen cosmopolitanism and the legitimacy of supranational polities (particularly the European Union [EU]), while it is less clear whether they entail denationalization. Further research is needed to improve the quality of independent and dependent variables in this area and assess the effect of international mobility and transnationalism outside the European context.