This chapter begins by highlighting how the history of sex-verification testing illustrates the extent to which many sport governing bodies have operated with a static and binary understanding of sex, creating a hostile environment for transgender athletes. It discusses the understanding that anatomical, chromosomal, and hormonal complexities involved in differentiating men from women make sex, like gender, a socially and historically-constructed concept. The general hostility of sport policymakers to those who transgress the sex binary is vividly illustrated by the history of sex-verification testing in sport. The "Stockholm Consensus" – a policy approved by the IOC Executive Committee prior to the 2004 Summer Olympic Games – marked an important turn with respect to the potential inclusion of transgender athletes. Overall, the emergence of such backlash demonstrates that recent growth in the acceptance of transgender athletes is tenuous; given the extent of efforts to roll back recent advances, advocates of transgender rights should not view progress as inevitable or irreversible.