In 1920, the Institut Jean-Jacques Rousseau organized an International Psychotechnics Congress, inaugurating the development of applied psychology in Europe (Carpintero, 2002): a highly significant event. We defend the hypothesis that psychology at the Institute, from its foundation in 1912 until 1929, developed above all with view to application. We will demonstrate this in four points (Hofstetter, 2010; Hofstetter, Ratcliff & Schneuwly, 2012). Fundamental psychology research was being carried out as a basis for application. As early as 1903, Claparède developed a powerful critique of associationism, highlighting the importance of interest and feeling in action, founding a functional psychology as the basis for work in the psychology laboratory at the heart of the Institute. The first director Bovet wrote works on fighting instinct and religious sentiment. Descoeudres addressed child development through questioning of children which Piaget would extend. The Institute was immediately established as a research and training center with laboratories in practical areas: Maison des Petits, special classes, medical-educational consultation, career counseling office, technopsychological advice, places where psychology was applied, enabling the creation of a multidisciplinary team. A dense international network was established, with students from all over the world, expanded by conferences on practical issues (bilingualism, peace, secondary education, inter-school correspondence, technopsychology) and an intense activity of publications and edition, mainly in education. In the aftermath of First World War, the commitment to new education was strengthened, thanks in particular to Ferrière: the Institute's international orientation evolved into internationalist commitment with the creation in 1925 of the International Bureau of Education, which saw itself as a hub for movements in favor of international and peace education (Hofstetter & Schneuwly, 2024), where applied psychology was regularly called upon.