This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The book have inspired attention to castrates that considered their gendered and sexual location although psychoanalytic accounts avoided actual, physical castration. Because of the overwhelming dominance of reproductive sex as legitimate, scholars across many disciplines have been attracted to castrates as individuals who embodied sexual and gender non-normativity. With their non-binary physicality, castrates offered historians, literary critics, and musicologists ways to think about how gender flexibility functioned in the past. In cultural terms, castrates were prone to conflate the physical and social, the libidinal and mercantile, and of course the masculine and the feminine around the castrated male body. The extensive, almost obsessive accounts of appearance, social propensities, and gender characteristics of castrated men reflected anxiety about the unreliability of the altered — disabled and transgendered — body.