Free improvisation is defined as a form of musical creation where preliminary decisions or pre-existing plans are kept to a strict minimum. Thus, the very idea of “rehearsing” free improvisation seems contradictory. However, long-lasting groups and collaborations that span over many years are not uncommon in the free improvisation scene. But, one might wonder, how do these musicians work together? How do they manage to balance the openness, spontaneity, and unpredictability of free improvisation with the unstoppable normalizing force of familiarity? In order to answer these questions, we need to understand what is at stake during the rehearsals of free improvisers. To this end, I will present the results of an ethnographic study conducted with several improvisation ensembles, showing the different functions rehearsals have for improvisers.