This chapter discusses the trans-formations of the category gender based on author's historical study about the medico-psychological problematization of ambiguous sex in German-speaking countries. The historical reconstruction of the medico-legal debate aims to dismantle the contingent and political nature of gender classification at stake in the negotiations about the boundaries between inter- and trans-sexuality. The chapter begins with an introduction to the German medical and legal context that structured the onset of the debate in the 1950s. Most post-war medical publications in Germany suggested that intersexual unlike the centuries-old term 'hermaphrodite' the term 'intersex' was only introduced in 1915 by the Berlin geneticist Richard Goldschmidt. It was developed in the context of animal experimentation on inheritance and designated Goldschmidt's particular theory of a genetic sex-determination based on a relational gene-concept. As early as 1900, physicians recommended that medical action be taken only in accordance with the intersexed patient's eventual sense of being male or female.