As a theory and practice concerning human reproduction, eugenics, from the Greek for “good/well” and “born,” is linked to sexology. This entry outlines and contextualizes the complex intertwining between sexology and eugenics in the first decades of the twentieth century, before eugenics became inextricably associated with Nazism. The contexts considered are Great Britain, where the word “eugenics” was coined, and Germany, where sexology was first named as a discipline and where Magnus Hirschfeld founded the first Institute for Sexology. The interplay of sexology and eugenics, including in the work of pioneers of sexual emancipation, such as Havelock Ellis, Auguste Forel, and Magnus Hirschfeld, reveals many contradictions—a “patchwork” of “progressive” and, with retrospective bias, now unacceptable ideas. It is recommended that close attention is paid to the historical context and to the hegemony of Social Darwinism in conservative and reformist discourses of the time in both Britain and Germany.