Many contemporary gender and politics scholars accept that there are theoretically coherent and defensible grounds for maintaining that, whilst undoubtedly a complicated one, some kind of relationship exists between women’s descriptive and substantive representation. The conceptual framework that has been commonly employed to hypothesize this relationship in practice is critical mass. Borrowed from physics, the term is usually understood to hold that, once women constitute a particular proportion of a parliament, ‘political behaviour, institutions, and public policy’ will be transformed (Studlar and MacAllister 2002, 234). As an argument for women’s descriptive representation, the concept has held great sway amongst feminist activists and gender and politics scholars over the last two decades or so. Recently, however, critical mass has come under sustained criticism.1 ‘Critical mass theory’ – the uncritical usage of the concept – posits a straightforward and simplistic relationship between women’s descriptive and substantive representation, one that is dependent upon a particular proportion of women being present in a political institution. Yet, empirical studies, in the UK and elsewhere, reveal multiple relationships between the proportions of women present in political institutions and the substantive representation of women; something which critical mass theory struggles to explain.