National audienceIn participatory democracy processes, the impact of citizens is often limited to the neighborhood level, without influencing the major urban transformations. However in Paris Rive Gauche, the associations came to have a significant impact on a large urban project. In reflecting on this exception, this paper shows that associations can impose their knowledge in the negotiations, despite the questions of scale and the economic dimension of the projects, from the moment they arrive to capture effects of political opportunities, to develop a capacity of counter countervailing expertise and to mobilize it in a double dynamic of cooperation and countervailing power. This articulation between logics of knowledge and power is proba...