The questioning of how public investments decisions are made leads to a two-level problematic. On the one hand, public decision makers have constrained resources that they have to use the best way they can. On the other hand, by choosing between alternative investments projects, decision makers are revealing priority choices between different stakes. So, decision makers have to face with two kinds of issues: spending with special care constrained resources and making the most acceptable

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... s they can. In France, for several years, cost benefit analysis has been used to take into account the first issue. The second issue has lead the legislator to rule public expression and stakeholders involvement. This has been a long process made of more or less successful trials. The purpose of this article is to examine how the use of cost benefit analysis interacts with the practice of public debate and stakeholders' participation. In Section 2 (after an introductory section), we will define more precisely principles of cost benefit analysis. In Section 3, we will see how stakeholders' participation has been progressively ruled in France. Then we will study the difficulties to conciliate cost benefit analysis and stakeholders' participation. At last, we will give some suggestions for conciliating legitimacy and rationality in public investments decisions making process.