The work of JCDecaux in Lyon is analyzed on the basis of interactions between the company and local authorities. Through the relations gradually established with the Lyon Urban Community, created in 1969, the firm JCDecaux had a part in setting up the urban agglomeration. The continuity in the relations between these two parties is used to investigate how the institutions are reworked that organize modern societies. Public institutions help structure the market and construct models of urban capitalism, while private companies help work out the framework for public authorities鈥?actions. This joint construction leads to ever more interactions and reinforces the interdependence that tends to legitimate a hybrid form of public/private action.