This chapter outlines the medical and scientific tests used to determine whether athletes were eligible to compete in women's sporting events in the twentieth century. Histories of sex testing in sports have tended to localise origins of tests in the games of 1950s, where in shadow of a Cold War successful female athletes from behind the iron curtain were accused of gender fraud. The first systematic at-event sex testing was introduced at 1966 European Athletics Championship, held in Budapest, and then taken up widely at other international sports events. Of course, sports organisations – including International Olympic Committee (IOC) and International Association of Athletics Federation (IAAF) – have reserved the right to insist on sex testing for athletes if specific accusations are made, or suspicions raised about their 'true' sex. A great deal of concern was also expressed about the need to protect female athletes from public criticism or negative evaluations of their physical appearance.