This essay reflects on the history of gender verification regulations aimed at female athletes in international sport and the justification for this practice under the rationale that it protects fair play in women’s sport. Beginning with the most recent guidelines for women participants proposed in 2011 following the controversy around the 800-meter world champion runner Caster Semenya, this analysis looks at the politics—gendered, scientific, and international in nature—of policing sex in women’s events through the use of scientific testing. To do so, the article traces the ideological underpinnings of fair play in Olympic sport and the development of international guidelines, providing a historiographic reading of the International Olympic Committee Medical Commission’s early work in this field during the 1960s, subsequent changes to this regulation, and their implications. In advancing this argument, I contend that to date the science aimed at preserving a steadfast binary has only provided evidence to the contrary. Science has yet to prove that female sex is definitive, and its recommendations have in part come to reflect that recognition. The recent revisiting of gender verification protocols nonetheless suggests the technocratic desire for a formation of sex that reflects binary constructions of gender, attesting to the continued power of heteronormative orders in this field and beyond.