This thought-provoking book takes a critical look at the relationship between biological sex and gender identity in the context of sport and considers the consequent implications for sport policy. It argues that biological sex is essential to an understanding of female athletes and sport participants, including the sex gap in sports performance, the history of female exclusion from sport, and the importance of the female sport category in relation to equal human rights, competitive opportunities and fairness for girls and women. This book features the work of leading international scholars and practitioners working in a variety of disciplines, including sport history, sport medicine, sport philosophy, sport physiology, sport policy and sport sociology, as well as equality, discrimination and human rights law. It examines the history of sex and gender in sport and introduces the fundamental anatomy, physiology and biomechanics of the female athlete, as well as the science of testosterone suppression, supplementation and disorders/differences of sex development (DSD). It also explores the theoretical debates around sex, gender identity and trans athletes in sport, including categorisation by sex, and considers their significance in terms of international and domestic sport policy, discrimination, fair competition and human rights. Centring the female athlete as the majority stakeholder in decisions regarding eligibility for the female category, this book is essential reading for anybody with an interest in women’s sport, social issues in sport, sport science, sport policy or gender studies.