This introductory chapter sets the scene for the book. It defines onward migration as a migration trajectory that involves extended stays in two or more destination countries and distinguishes it from competing and overlapping terms like stepwise and transit migration. Onward migration is a growing phenomenon within overall global migration dynamics, although statistics to document the scale and trends of this type of migration are scarce. The main aim of the chapter is to examine how onward migration and transnationalism are connected. This is achieved both by reference to a range of existing literature and by citing evidence from the succeeding chapters in the volume. We demonstrate both how transnationalism can shape onward migration and, the reverse, how different onward migration trajectories may result in different forms of transnationalism. In this latter context, we identify inter-generational, split, widening and re-routed transnationalism, again drawing on examples both from the book and from the wider literature. The final section of the chapter consists of a methodological discussion on the practicalities of researching onward migration and multi-sited transnationalism, in which the technique of multi-sited ethnography is discussed, critiqued and modified.