Telecommunications, transportation, energy and water supply networks have gained crucial importance in the functioning of modern social systems over the past 100 to 150 years. The development of these networks and the economic, social and environmental issues associated with it are the subject of Sustaining Urban Networks. The authors analyze challenges to the expansion of access to and use of network-supplied services, as well as challenges associated with such expansion. Far from arguing that expansion is always positive, some of the authors argue that universal development of some networks may prove to be unsustainable. Analyzing the relations between cities and networks is crucial to discussions of the sustainability of networks and of cities. On the one hand, cities have been, and are, increasingly, dependent upon the smooth functioning of a host of technological networks ; on the other hand, cities are where technological, economic and social innovations originate, that support the initial development of networks. The functional dependence of cities on infrastructure systems, the social dynamics associated with the initial expansion of a new network in a city, and issues of social/spatial access to basic utility services are analyzed in the chapters of the book.