Proximity and agglomeration, two understanding keys of city

International audienceIn this paper we tried to understand how the game played by the different proximities is at the core of the creation and the existence of cities, on the basis of economics of agglomeration and economics of proximity, using costs analysis. We showed that both approaches bring partially interesting responses to the question of the existence of cities and the polarization of human activities, but that a more general and interesting explanation could be driven from their combination. Given that, we described the processes of urban agglomeration, but also the relations between cities, by means of proximity tools, modified to take into account the main principles of urban agglomeration or remote exchanges.Our proposition takes into account the most interesting characteristics of both approaches and offers a new perspective of understanding of the urban systems and their relations. It presents the advantage to handle and to bring an explanation to intra-urban level phenomena, including the limits of the processes of urban agglomeration in terms of conflicts and congestion, and to leave opened the interpretation of the existence of city networks and global metropolises or global cities (Bourdeau-Lepage and Huriot, 2005) and of their global impact in terms of growth. We intend to forge links between the borders of both paradigms - agglomeration and proximity economics - and to show that their crossed pollination allows to bring explanatory essential elements in the understanding of contemporary urbanization processes