This article presents an overall reflection on the libraries that were assembled in scientific institutions in France and Italy in the eighteenth century using case studies and comparative approaches. It focuses in particular on five scientific academies (located in Turin, Florence, Paris and Brest) and two Florentine institutions, the Museo di Fisica e di Storia Naturale and the Santa Maria Nuova Hospital. Decisions made regarding library premises, book procurement policies, catalogue publications and whether or not to open to the public were all investments that demonstrated the role of the written culture in the identity of scholarly communities, including those (such as the naval and surgeons’ communities) whose members had long been seen as professionals firmly rooted in a manual practice that was detached from theory. This article thus shows how libraries participated in the institutionalisation of scientific activities, the definition of professional knowledge and the formation of scholarly collectives.