Sociology and economics tend to focus more and more on the intermediaries involved in economic and social relations, in the shape of distributors, matchmakers, consultants, and evaluators. Once they are distinguished according to their forms, their types of intervention and their effects, the intermediaries are a helpful category in order to study the social organization of markets as well as the changes that operate on them, especially regarding the social and economic values of goods, individuals and organizations. We discuss in the first section the link between intermediaries and information, through an analysis of the functions they fulfill that may explain their emergence, as well as the opportunistic behavior of intermediaries in relation to information flows. In the second section, we adopt a more pragmatist perspective on issues of valuation mainly based on ”economics of convention”, which emphasizes the collective dynamics of valuation. We show how intermediaries contribute to de!ne valuation through their different activities and foster valuation frames that can improve the coordination of actors, but also reorganize the markets in different ways. We suggest an analytical distinction between the distribution, the temporality and the generality of the frames, and raise the issue of the valuation power of market intermediaries, their legitimation and the