This chapter discusses definitions of migrant transnationalism, a concept that captures the cross-border ties and everyday practices migrants and their loved ones may maintain across international borders. Transnational connections—such as emotional bonds, financial support, and cultural practices—demonstrate how migrants’ lives are often shaped across multiple locations, creating multi-sited or transnational lifeworlds. While migrant transnationalism is widespread, as evidenced by the global scale of remittances, it is not experienced equally, rather there are variations in how, why, and to what extent migrants engage transnationally. This chapter argues for the idea of applying a transnational lens to migration studies, and discusses the value of this approach, while also reflecting on the need to limit what is understood as ‘migrant transnationalism’. The chapter ends with an outline of the book’s main thematic areas and its structure.