Readers are introduced to the unusual and controversial history of sex determination technologies and policies in sport. Major sport organizations, most importantly the International Olympic Committee and the International Association of Athletics Federations, from the 1960s onwards required female athletes to undergo various examination procedures. This includes the so-called “sex tests” that were used in order to verify the legitimacy of the athlete’s biological sex necessary in order to compete as a woman in sport. This chapter first contextualizes the existence of sex tests in terms of the historical relationship between gender and power in sport. Then the complex relationship between the science and technologies is described as being used to “test” female athletes, which includes an analysis of the various social agents that created the policies of sex determination and applied the specific tests. Finally, “sex testing” is discussed based on the current, various social and legal challenges against such practices. A summary is provided on the current movement to make sport more trans-inclusive at both the grassroots and higher levels will have on sex determination policies. Finally, emerging technology, not yet in use, is expected to be able to refine the line established between the sexes; however, this is not expected to resolve the issue.