Legislative member organisations (LMOs)—such as caucuses in the U.S. Congress and intergroups in the European Parliament—exist in law-making bodies around the world. Unlike parties and committees, LMOs play no obvious, predefined role in the legislative process. They provide legislators with opportunities to establish social networks with colleagues who share common interests. In turn, such networks offer valuable opportunities for the efficient exchange of policy-relevant—and sometimes otherwise unattainable—information between legislative offices. Building on classic insights from the study of social networks, the authors provide a comparative overview of LMOs across advanced, liberal democracies. In two nuanced case studies of LMOs in the European Parliament and the U.S. Congress, the authors rely on a mix of social network analysis, sophisticated statistical methods, and careful qualitative analysis of a large number of in-depth interviews.