The evaluation of ecosystems and biodiversity has become an important field of investigation for economists. Although their interest has been largely motivated by the search for arguments in favour of broader conservation policies, both the methods and the meaning of the results remain controversial. This article aims at clarifying the interest and limitations of these works, by revisiting a number of issues, such as the economic qualification of the services that human societies take from biodiversity and ecological systems in general, the specificities of their contribution to human well-being and the consequences of a valuation of biodiversity based on ecosystem services. We conclude with a discussion of the purposes of evaluations: improving public policies or creating new markets?