From its obscure mid-twentieth-century origins in clinical assessments of intersex individuals, "gender identity" – understood as an inner sense of self that may or may not align with one's natal sex – has become central to the public and private culture of selfhood throughout the Western world. The category has been enacted in laws and regulations, encoded in organizational policies and routines, and embedded in everyday ways of thinking and talking. It has introduced a novel principle of social classification, created new possibilities for personhood, and come to rival or redefine the category of sex itself. These developments have provoked intense controversy. Disputes rage about the legitimacy of medical interventions for gender-dysphoric youth, the meaning and limits of gender self-determination, and the terms governing access to women's spaces and activities. Rogers Brubaker treats this hot topic in a cool manner, bringing much-needed analytical distance and conceptual clarity to a discussion dominated by polemics. Renowned for his foundational analyses of basic social categories, Brubaker sidesteps intractable debates about what gender identity ultimately is in order to show how the category actually works in medicine, law, data-gathering, and education. And he challenges prevailing understandings by demonstrating that despite its obvious instrumentalization by the right, most spectacularly in the US, the controversy over gender identity cannot be understood solely through a left–right lens. Wide-ranging and rigorous, this book will inject a healthy dose of reason into one of the most troubled and acrimonious debates of our time.