The vast majority of sports competitions are divided into two, mutually exclusive female and male categories, which presents significant obstacles to those whose bodies and / or identities do not align with this binary sex division. For decades, international sport governing bodies have had in place regulations that enforce the binary sex division by requiring those athletes who are registered to compete in the female category to submit to “sex tests.” In other words, sport regulators have subjected athletes to various kinds of medical examinations in order to determine their eligibility to compete in the female sporting category. The tests that have been used for this purpose have changed over time – ranging from physical exams to chromosome screening to analysis of hormone levels – and sex testing in general has been controversial, because there is no single test that can determine someone’s sex. Medicine has long recognized that “female” and “male” are not, empirically speaking, mutually exclusive categories. Indeed, the regulations and sex tests that sport governing bodies have used to enforce binary sex categories have resulted in the exclusion of many intersex athletes or athletes who have differences of sex development (DSD) and transgender athletes from competitive sports. This timeline summarizes the regulations that have been used by the International Olympic Committee (IOC) and the International Association of Athletics Federations (IAAF) to enforce binary sex categories in the Olympics and international athletics. The IOC and IAAF were chosen because these two sport governing bodies have been the most influential in shaping the regulation of binary sex in sports more broadly.