The past two decades have witnessed a sea change in migration scholarship. Most scholars now recognize that many contemporary migrants and their predecessors maintain various kinds of ties to their homelands at the same time that they are incorporated into the countries that receive them. Increasingly, social life takes place across borders, even as the political and cultural salience of nation-state boundaries remains strong. Transnational migration studies has emerged as an inherently interdisciplinary field, made up of scholars around the world, seeking to describe and analyze these dynamics and invent new methodological tools with which to do so. In this review, we offer a short history of theoretical developments, outlining the different ways in which scholars have defined and approached transnational migration. We then summarize what is known about migrant transnationalism in different arenas—economics, politics, the social, the cultural, and the religious. Finally, we discuss methodological implications for the study of international migration, present promising new scholarship, and highlight future research directions.