Harry Benjamin (1885-1986), a German-born endocrinologist, was a pivotal figure in the development of transgender medicine. He was physician to transgender pioneers such as Christine Jorgensen, the 1950s "Ex-GI" turned "Blonde Beauty" media sensation, and in turn, she and other collaborators helped to shape Benjamin's influential 1966 book, The Transsexual Phenomenon. Alison Li's much-needed biography of Benjamin chronicles his passion for hormones and his lifelong interest in sexology. Drawing from extensive research in archival documents, secondary sources, and interviews, Li tells the story of Benjamin's early ventures in gerontology and his later work with over a thousand transgender patients. Benjamin's contributions to treatment, education, research, and networking helped to create the institutional foundations of transgender medicine. Moreover, they set the stage for a radical reconsideration of gender identity, challenging us to reflect upon what it is to be male or female and to envision moving beyond these long-held categories.