This introduction explores reasons for the continuing debate on the subject of transnationalism and persistent scepticism about the significance of the topic. The basis for such disagreements has to do less with the actual existence of the phenomenon than with methodological shortcomings that led to its overestimation in the early literature and the conceptual failure to distinguish between cross-border activities conducted by major institutions and by private actors in civil society. I explore these various problems seeking to clarify the actual scope of the phenomenon of transnationalism and its novel character. Despite recent findings that point to limited numerical involvement of immigrant groups in transnational activities, the latter remain significant because of their prospective growth and their impact on both immigrant adaptation in receiving countries and the development prospects of sending nations and communities. The evidence presented in the following articles document in detail these various aspects and indicates the multiple forms adopted by this phenomenon among immigrant groups in Europe and the United States.