The modern economy is characterized by intensive use of knowledge. Considering the fact that knowledge theoretically has very low use costs, returns should be particularly high. Yet this does not seem to be the case, probably because of real use costs, which the author attempts to identify and explain. Costs of access, acquisition and use of knowledge slow down the circulation of information, and new technologies are only of limited help. This article describes the highest costs and various ways of getting round them. It foresees increasing rationalization in the use of knowledge, especially in the as yet untapped area of tacit knowledge.