This article highlights the trials and tribulations of citizenship in a world of increasing mobility and diversity. The discussion is divided into three parts. Section I provides a concise overview of citizenship's multiple meanings and interpretations. Section II constitutes the bulk of the discussion. It begins by exploring questions of membership acquisition and transfer, which legally determine ‘who belongs’ within the boundaries of a given political community, either by birth or naturalization. It then assesses three recent developments: the growing recognition of dual nationality; the revival of debates about involuntary citizenship revocation; and the ‘cultural turn’ in citizenship discourse, which often makes inclusion in the body politic more difficult for those deemed ‘too different’ from the majority community. Section III charts the major challenges and opportunities facing citizenship in the twenty-first century.