Competition or complementarity ? An investigation carried out by the Observatoire de l'économie du livre, explains the links - complex and not often studied - between purchase of books and borrowing in library. It at first confirms the weight of sociodemographic factors in the practice of both ways of access to the book. If the users'discourse often refers to the sharing of the roles, from their point of view, which expresses a complementarity between both practices, there is all the same areas of competition, a fortiori under the economic point of view. With the collected data, this competition does not seem to appear in massive transfers from purchase to borrowing.