Between scientific expertise and power games: the Green and Blue Infrastructure of the Grenelle Environment Forum. In this paper we investigate the deliberative process charged with defining an integrated conservation policy within the framework of the French Grenelle Environment Forum, namely the Green and Blue Infrastructure ("trame verte et bleue" [TVB]). We point out the central importance of scientific expertise in the social process leading to the representation of TVB and the definition of a framework for public action. The hybrid forum in charge of its characterization did not function in isolation. The TVB definition and the resulting public action framework are dependent on social interactions and networks, historical constraints or supranational legal frameworks, which transcend the participatory arena and question its autonomy. We emphasize the importance of the place given to interdisciplinary academic expertise as determinant in creating sense for the group and orienting it toward a "common cause".