Centratity and Concentration-Central place theory has « obsessed» geographers during the past 50 years. The contours of the christallerian «dogma» are considered, and research in this tradition is assessed. Central place theory focusses on the centrality of urban functions. The concentration of population within urban networks must also be considered and, in particular, the long term évolution of the distribution of cities according to size. An index of hierarchical structuration is used hère to this end. The index is applied to the évolution of the French urban structure between 1831 and 1911.