The study of the evolution of an urban system raises a number of practical questions, as to the operational definition of the components of the system as well as to the incorporation of the factor time in the analysis. In this chapter the fo cuss is on the urban centres, the nodes of the urban system. One of the first pro blems to be solved now is the definition of such a node. It has been argued above that population size can be used as an approximation of the complexity of the spatial economic structure of a region. The acceptation of this notion makes it possible to study the urban system...