This introductory chapter situates the book in relation to the key theoretical, social, and historical issues that it addresses. It also situates the book in relation to existing historical narratives about gender verification in international sports and places it in the context of the wider theoretical and historical questions about the ontology and epistemology of sex difference. In so doing, it highlights what is at stake when it comes to gender verification, both theoretically and empirically, not only in sport but in society more generally. Firstly, it considers challenges that pertain to writing histories of gender verification and explains why this book offers a genealogical alternative. Secondly, it offers a conceptual history of sex, gender, and the body in relation to dominant socio-scientific configurations as well as feminist configurations of sex difference. Finally, it foregrounds the feminist lens from which the book is written and the Butlerian theoretical framework that it adopts to analyse gender verification; the chapter provides a structural outline for the subsequent chapters.