French unions do not constitute places of a high expertise in public policies today despite the fact that they are members of a number of joint or tripartite institutions (such as economic, social and environmental national Council) responsible for producing diagnostics and recommendations in the social field. If, in the decades 1970-1980, proposals and diagnostics carried especially by the CFDT have influenced the definition of specific policies in the world of work (for example the Auroux laws, but also the Roudy law about wage equality) or social policies, the trade union’s ability to act like a critical actor appears now much reduced. The chapter first seeks to explain this relative weakness, in relationship with several factors (links with universities, research organizations, etc.). Second, it examines those specific areas in which trade union’s expertise exists and has some influence in the definition of public action (educational and industrial policies,). Drawing from an original dataset, this chapter reflects on the forms taken by the unions’ public policy expertise and the influence it can exert.