This post-doc project aims at modelling at micro-level the expansion of firm’s networks within a system of cities. Multinational firms are developing their networks in an heterogenous geographical environment that they contribute to transform. Their networks are increasing according to transaction rules. Their location concentrates in cities, whose agglomeration economies tend to optimize the interactions between firms inside and between cities. Multinational firms being inserted in the urban economy are modifying the local socio-economic networks and enhance the influence of the city in its system.