Pre-print, Working paper: This is the text of the opening lecture to the "Journées de l'Association Française de Science Economique", held at Rennes, (2004). In accordance with the rules of the exercise, the article provides a broad assessment of the difficulties and usefulness of cost benefit analysis, as a tool for to-day public decisions. The analysis both emphasizes the changes of the institutional environment in which public cost-benefit analysis takes places, the merits of its "software" (its foundations), and of its "hardware" (its implementation techniques).