Gender quotas have spread rapidly around the world in recent years. However, few studies have yet theorized, systematically or comparatively, variations in their features, adoption and implementation. This article surveys quota campaigns in Western Europe, North America, Australia and New Zealand. It proposes that one or more sets of controversies influence the course and outcomes of quota reforms. These revolve around (1) competing principles of equality, (2) different ideas about political representation, and (3) various beliefs about ‘gender’ and its relation to other kinds of political identities. The article draws on these distinctions to identify four broad models of political citizenship that determine the kinds of quota policies that are pursued and their prospects for bringing more women into political office.