The interactions which occur between individuals depend on the distance between them and the ensuing transportation and communication costs. A central position allows for the minimization of these costs: activities which are not tied to resources found only in particular places seek this type of location. For central place theory, cities are fundamentally market places, i.e. central locations for the neighbouring areas, and urban networks hierarchized Systems of communication. Centrality is not only functional: it explains the symbolic role of cities. Recent progress in communication technology transforms the traditional structure of urban networks and is responsible for the contemporary trend towards metropolization.