This article explores "determining gender," the umbrella term for social practices of placing others in gender categories. We draw on three case studies showcasing moments of conflict over who counts as a man and who counts as a woman: public debates over the expansion of transgender employment rights, policies determining eligibility of transgender people for competitive sports, and proposals to remove the genital surgery requirement for a change of sex marker on birth certificates. We show that criteria for determining gender differ across social spaces. Gender-integrated spaces are more likely to use identity-based criteria, while gender-segregated spaces, like the sexual spaces we have previously examined (Schilt and Westbrook 2009), are more likely to use biology-based criteria. In addition, because of beliefs that women are inherently vulnerable and men are dangerous, "men’s" and "women’s" spaces are not policed equally—making access to women’s spaces central to debates over transgender rights.