Prof. Lena SANDERS, CNRS, Paris, France. Cities evolve at different rhythms, some growing, other stagnating or even declining. The hypothesis is that these differences are the result of different abilities to valorise a position in a system of cities. The interactions between cities play then a driving role in their evolution. First I will briefly present a family of models based on that principle, the SimPop models. Then I will use particular one, the EuroSim model which concerns the evolution of the European cities during the period 1950-2050, in order to show and to discuss how the cities’ networks are built up. According to its economical specialisation, a city can be involved in one or several networks which are evolving through time according to a combination of exploration and exploitation principles. At last a comparison of the consequences of different scenarios (according to the existence or not of border constraints on the exchanges between cities of different countries) by 2050 is presented.