For regional organizations (ROs) as geographically defined entities, questions of membership often raise moral questions about the very foundations of regional identity. To date, comparative approaches to the role of norms in the politics of RO enlargement are a missing piece in the regionalism literature. This paper assumes that enlargement practices are shaped by discourses about legitimate actorness. Drawing on the concept of Bstandards of civilization,^I argue that prospective RO members are judged against a set of norms-the standard of membership-which constitute basic ideas about the identity of the regional international society. As evidence from the Spanish and Greek accession to the European Community and the accession of Myanmar and Cambodia to ASEAN shows, this standard is not a static catalogue of cultural values, as existing accounts suggest, but develops in response to accession requests which trigger normative crises among the existing member states. In such situations, RO actors may argue for the inclusion of new norms in the standard of membership by drawing on Bcognitive priors^. Enlargement processes thus reveal a dialectical relation between regional norms and boundary-drawing; while a regional standard of membership informs the redefinition of an RO's boundaries, the accession of new candidates also transforms that very standard.