In recent years, the concept of boundaries has been at the center of influential research agendas in anthropology, history, political science, social psychology, and sociology. This article surveys some of these developments while describing the value added provided by the concept, particularly concerning the study of relational processes. It discusses literatures on (a) social and collective identity; (b) class, ethnic/racial, and gender/sex inequality; (c) professions, knowledge, and science; and (d) communities, national identities, and spatial boundaries. It points to similar processes at work across a range of institutions and social locations. It also suggests paths for further developments, focusing on the relationship between social and symbolic boundaries, cultural mechanisms for the production of boundaries, difference and hybridity, and cultural membership and group classifications.