Around 1900, interest in adolescence as a separate and crucial phase in human development increased among psychologists, educators and youth workers in the western world. This paper reviews the relation between adolescence and sexuality in the early twentieth century from a Dutch perspective. In the 1920s pedagogues started to study adolescence. The Amsterdam professor Philipp Abraham Kohnstamm was a pioneer in developing a pedagogical approach (instead of a clinical approximation) to understand adolescence and sexuality. For Kohnstamm, sexual health education had to be part of moral teaching. He stressed the importance of sex education at an early stage, for body and mind are connected to such an extent that a separation will result in the depersonalisation of man. Kohnstamm's work reflected the ideas of the European intellectual elites of his time regarding the sexual morals and manners of the working class.