This editorial introduces a special issue and its different contributions. We provide an overarching background for the special issue by situating it in the context of the European Union integration process and through a general discussion of the role of intermarriage in establishing links across borders. The importance of studying union formation, that is, marriages between Europeans in the context of the European Single Market is discussed. We argue that marriages between Europeans can promote the development of trans-European solidarity ties and may in this regard contribute to European identity formation. This article introduces the goals of the different research pieces in the special issue and outlines the overarching issues that are demonstrated by the papers collectively. We conclude that European binational marriages are still rather rare and that binational European couples are not a homogenous group. Our findings point towards the continued relevance of local geography, migrant history and patterns, as well as cultural and linguistic similarities for partner choice in a unified Europe. We call for a more integrated approach and a clear research agenda setting for the further study of union formation and dissolution in the European single market.