In this chapter, we introduce and explore approaches to migrant transnationalism, asking how cross-border connections were understood at the time the term first gained traction in the 1990s. The first section explores the ‘what’ of migrant transnationalism and traces the field’s foundations in interdisciplinary migration studies, emphasizing evolving approaches, and including tangible and intangible aspects of migrant transnationalism. The second section focuses on the geographies of migrant transnationalism–the ‘where’–places and spaces involved, discussing the two most used spatial concepts: transnational social spaces and transnational social fields. The third section examines the ‘who’–or the actors in transnational social fields—migrants, non-migrants, families, wartime migration, and religious actors. This section also includes considerations on time and the life course, such as whether transnational ties endure or fade over time?