Because a city cannot be conceived of as an isolated system but is always part of a system of cities (Berry, 1964), interaction between cities is an essential component of the dynamics of urban systems. Each town or city is a persistent and relatively autonomous entity whose evolution is influenced or limited by the towns and cities in the same interaction networks (Pred, 1977). It has been demonstrated that competition between towns and cities for resources and growth is the main driving force in the dynamics of systems of cities, which also explains the pervasiveness of their structure (Pumain, 2000). Most systems of cities link neighbouring towns and cities located on the same regional or national territory. But cities also have relationships with more distant competitors, especially in specialised networks, which is more and more the case in the current context of an increasingly global economy. Therefore, the hierarchical organisation of cities in systems of cities is no longer inclusive (if it ever was), since the multiple links connecting towns and cities can go anywhere. However, many of the relationships between urban actors are recurrent or use the same communication channels. Recurrent interaction patterns shape the structure of urban systems. This structure is universally characterised by strong hierarchical differentiation (Pumain, 2005). Several orders of magnitude separate the importance of towns and cities, in terms of population, gross product or influence. This importance can be defined by the relative position of a given city in interaction networks (centrality, betweenness), as well as by the urban attributes representing the cumulative effects of that position over time. Interaction flows in turn reflect (or are induced by) this structure of the urban system, since they are generated by the attributes of one city rather than another’s.