This chapter explores the cases which have challenged the current practices of inclusion and exclusion in relation to sex, drawing upon instances when the divisionary lines between men and women in sport have been brought to challenge by women. The balance between inclusion and exclusion in sport on the basis of sex appears to be most precarious in the areas of mixed-sex sports since this directly challenges an integral structure of sports, which is sex segregation. Exemption clauses that support the segregation of men and women raise important points about the biological and social construction of men and women in the context of inclusion and exclusion in sport. From a social perspective, traditional gender roles and differences in absolute strength have resulted in misconceived approaches to strength training for women. Physiology perspectives do not ignore the fact that social factors can also account for body fat and haemoglobin differences in men and women.