Abstract Today many parties interact with members and supporters outside their national borders. One way parties do so is by establishing transnational party branches. However so far, there is a lack of theoretical and empirical research exploring this transnational aspect of party activity. This paper provides a first insight into why parties develop transnational branches, and how it affects their organization. It argues that the development of party branches abroad differs across countries due to the incentives provided by the national legal framework on voting and donations from abroad. In turn, the role and functions of the transnational branches vary depending on this legal framework. Looking at the two transnational branches of the two mainstream parties in France, Germany and the United Kingdom, this paper first briefly outlines the different national legal frameworks and the scope and size of transnational party branches. It then focuses on how they are organized, their role and function, and how this shapes their relationship to their homeland party. This explorative research is mainly based on legal and secondary data. The paper finds that the parties studied differ substantially in location, number and membership size for their transnational branches. Furthermore, the organizational links and the control by the national party over transnational party branches is the highest where parties have the most to gain in terms of votes in national elections and donations.