In the first two decades of the twentieth century, the figure of the adolescent (Jugendlicher) was introduced into public discourse in the German-speaking world. The adolescent soon became an epistemic object for the still loosely defined field of psychology. Actors in the slowly differentiating scientific field of youth psychology were primarily interested in the normal development of adolescent subjects and sought out new materials and methods to research the inner life of young people. In order to access this inner life, (...) they turned to the interpretation of diaries and other self-descriptions. This article takes up the questions of how diaries were used in the scientific context of psychology, and how diary writing was psychologically interpreted and theorized. The theoretical and methodological contexts of psychological knowledge production grouped around the subject of the diary will be examined in keeping with Hans-Jörg Rheinberger’s concept of historical epistemology. This analysis is carried out by using the example of three central actors who were in conversation with each other during the 1920s and 1930s: the developmental psychologist Charlotte Bühler (1893–1974), the psychologist and founder of personalistic psychology William Stern (1871–1938), and the youth activist Siegfried Bernfeld (1892–1953), who was influenced by Freudian psychoanalysis. (shrink)