This chapter examines the origins and legacy of sexology – the scientific study of sexuality – in the modern world. First consolidated into a coherent programme in the late nineteenth century, sexology has its roots in the re-organization of knowledge about nature in the frameworks of taxonomy, evolutionism, and race. A pervasive preoccupation with heredity gave rise to powerful eugenics movements around the world. The interest in controlling variability and unlocking the secrets of the soul generated parallel developments in biomedicine, especially psychoanalysis and endocrinology. Sex experts worldwide converged in diagnosing cultural signs of homosexuality for the purpose of national modernization. As the centre of gravity in sexual science began to shift from Europe to North America, researchers gave growing support to the sex/gender distinction and redefined the meanings of normality. In the waning days of hereditarian theories, the rise of cultural anthropology coupled with a renewed scientific investment of colonial powers to reverse hierarchical templates of sexual practices and norms emanating from the metropoles. A public health crisis (HIV/AIDS), social movements (gender and sexual minority rights), and the systematization of research protocols (bioethics) shaped a comeback of biological sexology in the closing decades of the twentieth century.