This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The book explores the main composition of ideas relative to the contemporary psychiatric and sexological literature, and the most important theoretical and methodological aspects of the 1905 edition of the Three Essays. It elaborates a fundamental tension in Freud's thinking concerning human sexuality as both inherently object-related and inherently independent of this relatedness. The book situates the Three Essays in the larger context of the modernization of sexuality by focusing on two pioneers of sexual theory, Richard von Krafft-Ebing and Albert Moll. It focuses on the ambiguity of Freud's basic clinical and conceptual vocabulary that ultimately resulted in the normalization of sexuality by psychoanalysis. The book highlights the role of the perversions in Freud's Three Essays and confronts Freud's patho-analytic approach with Lacan's reference to a perverse structure that seems to (re-)introduce the idea of the pervert subject having a different identity.