Author Manuscript, Published in "complexity Perspective in Innovation and Social Change

Introduction Cities 1 are a major form of the material, social and symbolic organization of societies. They are persistent and adaptive structures which fulfill a variety of social functionalities: habitat, production, services, political control over people and territories, as well as technical and symbolic mediation between nature and culture, groups and individuals. For a long time, and especially since the first industrial revolution, they have been both the spaces where most innovations occur and spaces for which most innovations are designed (chapter 8). Location in space and time has to be considered as a very important feature for these systems, since societal evolution is much more rapid than the biological one. Being adaptive, cities alter through a variety of intentional actions that may produce non-intentional persistent features: urban structures are produced partly through design and planning, partly through self-organization. What is analyzed in this chapter is a conception of urban systems as complex systems. We develop a theory of the organization of cities and systems of cities as multilevel networks, including two main observable levels, where specific emergent properties can be observed. Compared to the earlier seminal conception by B. Berry (1964) of " cities as systems within systems of cities " , we emphasize a multilevel organization perspective and specific dynamic features that produce this structure. We also add an evolutionary perspective, which is based on observations about the way cities co-evolve within urban systems, through a variety of social interactions. Empirical evidence of the corresponding patterns and processes is provided for three main styles of urbanization, exemplified in four different parts of the world.