The success of the notion of "project logic" parallels the critique of the classical model of city production. A new model of action, based on the co-production of the city, emerges. This model calls for a renewal of practices in the preliminary phases of projects. The capacity to go beyond predefined needs and solutions, on the one hand, and the collective definition of projects, on the other, are emphasised. Thus, the elaboration of projects is paramount and the piloting of preliminary phases is at the forefront. Nevertheless, even though the capacity to conduct an activity depends on the understanding of that activity, our knowledge of the building of projects and its preliminary phases is limited. Thus, this thesis intends to explore the project building activity with two case studies : the B line of the Strasbourg tramway and Montpellier's Odysseum. Two types of results emerge : first, a modelisation of the project activity in urban planning. This model, built on the notion of "project situation", structures a conceptual frame that allows us to go beyond the indertemination of elaboration processes. Secondly, three main characteristics of the key steps of piloting are defined : the logic of action that structure the piloting of projects ; project management ; the political piloting of urban projects. Moreover, these results allow us to reflect on issues related to urban projects and urban co-production and innovation
