Functional foods have swept across France and Europe since the late 1980s. Margarines enriched with omega 3 or phytosterols, yoghurts enriched with bifidus or vitamin-rich food supplements... these foods with health claims have reopened the boundary erected over the course of the twentieth century between the markets for medicines, whose primary function is to treat the sick, and those for foods, whose primary function is to meet the nutritional needs of healthy people. They thus raise many issues for regulators, producers, distributors and consumers. Are they health products that can usefully contribute to the prevention of chronic diseases or marketing manipulations that are not of any health interest? Should they be regulated and marketed as drugs, as food or as a separate category? Drawing on significant contributions in economic history and sociology, this article analyzes the consequences of the classification of these products as medicines, foods or dietary supplements on their valuation and the structure of their markets. It also focuses on the boundary work in which industry and regulatory actors engage in order to “(re)classify” these products, and thereby “(re)structure” the markets in which they circulate. The first part of the article describes the process that led France to establish a boundary between the previously intertwined markets for medicines and foods over the course of the twentieth century, which has been blurred by the appearance of functional foods since the beginning of the 1980s. The second part deals with the efforts of French and European regulatory authorities and industry actors to renegociate these boundaries between the markets for medicines and foods bearing nutritional and health claims. We conclude this second part with a discussion of the consequences of this reframing for the reorganization of markets and the reevaluation of the health benefits and commercial advantages of these products.