In response to the mass globalization of the twenty-first century and associated migration, a recent boom in social-scientific research has analyzed various manifestations of 'binational', interreligious and interracial romantic relationships in the present and recent past. This special issue seeks to historicize this research by drawing on key case studies from around the world and across time and building on relevant historiography and theoretical literature. It seeks to chart how intermarriage and related relationships took shape: who participated in these unions? How common were they, and in which circumstances were they practiced (or banned)? With a global, diachronic and interdisciplinary perspective, we also aim to question some of the categories behind these relationships. Central to these issues, we argue, is the question of boundary formation. Here, we draw on social-scientific research that has emphasized multiple boundaries involved in the creation of identity and groups. We also highlight the intersectionality of those boundaries, meaning that notions about ethnicity, religion, gender and social class often overlap and intersect in various ways when it comes to relationships. Contributions to this collection tap a range of related questions, such as how did geographical boundariesfor example, across national lines, distinctions between colonies and metropoles or metaphors of the 'East' and the 'West'shape the treatment of intermarriage? What role have social and symbolic boundaries, such as presumed racial, confessional or socio-economic divides, played? To what extent and how were those boundaries blurred in the eyes of contemporaries? How have bureaucracies and law contributed to the creation of boundaries preventing romantic unions? Romantic relationships, we suggest, provided a key test case for boundary crossings because they brought into sharp relief assumptions not only about community and assimilation, but also about the sanctity of the intimate sphere of love and family.