One of the twentieth century’s most controversial sexologists—or “fuckologists,” to use his own term—John Money was considered a trailblazing scientist and sexual libertarian by some, but damned by others as a fraud and a pervert. This is the first book to contextualize and interrogate Money’s writings and practices across his three key diagnostic concepts, “hermaphroditism,” “transsexualism,” and “paraphilia.” The book offers a multidisciplinary critique of the tensions and controversies that engendered and followed from Money’s work. He invented the concept of gender in the 1950s, yet fought its uptake by feminists. He backed surgical treatments for transsexuality, but argued that gender roles were set by reproductive capacity. He shaped the treatment of intersex, advocating experimental sex changes for children with ambiguous genitalia. He pioneered drug therapy for sex offenders, yet took an ambivalent stance towards pedophilia. In his most publicized case study, Money oversaw the reassignment of David Reimer as female following a circumcision accident in infancy. Heralded by many as proof that gender is pliable, the case was later discredited when Reimer revealed that he had lived as a male since his early teens. Bringing Money’s ideas into dialogue with both the theoretical humanities and the history of medicine, the book also addresses Money’s lesser-known work on topics such as animal behavior, cybernetics, brain development, and the philosophy of science.