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The valuation of biodiversity and ecosystem services is the subject of many studies and extensive debate. To overcome problems of principle, the real meaning of economic evaluation must be clarified in order to identify its advantages and limitations. An appropriate analysis of these services implied extending the concept of value and has given rise to a plurality of methods that may be strongly challenged. The purpose of this paper is to clarify some terms of these debates. There are reasons for being reluctant to give a price index to Nature’s services, but its conservation entails costs, at least opportunity costs. Most valuation methods rest upon individual preferences, either revealed by concrete behaviours, or stated during interviews, and this is clearly an issue for items agents are not familiar with or do not perceive clearly the benefits. Finally, valuing biodiversity through ecosystem services might lead to underestimate some major parts of what constitutes biodiversity social value. These issues have to be replaced in relation to the very different objectives that may further the actual valuations. The paper concludes that valuation can offer quite a valuable contribution to conservation policies, but since the relation between biodiversity, ecosystem services and human well-being remains poorly understood, that it is a far too important stake to let economists alone in facing these puzzles.