In recent years, new perspectives on sexuality have been welcomed in psychology. Among the various phenomena addressed in this direction, the phenomenon of transsexuality stands out. The aim of this book is to present the psychoanalytic contributions of Freud and Robert Stoller to understanding the emergence of sexual identity. To this end, Freud's hypotheses on the formation of sexual identities and Stoller's hypotheses on the development of transsexuality will be presented, as well as discussions on gender and the psychopathologies surrounding discussions on this subject.