The chapter develops a theory that encompasses as far as possible the existing state of knowledge in urban sciences for identifying regularities at the world scale and enabling worldwide comparisons of urban systems. Relying on the specific concepts in geography that consider cities as complex systems which self-organize and co-evolve in systems of cities through their socio-spatial interactions, it provides an explanation of the emerging common properties of these systems, including their hierarchical organization and functional diversity. Major driving processes are the creative emulation in urban competition and the spatial diffusion of innovation that are renewing opportunities and challenges to urban stakeholders. Cities and systems of cities acting as socio-spatial adaptors can be considered as expressing a collective territorial intelligence because of the resilience of urban systems over long time periods. According to variations in space and time such generic processes when contextualized and taking into account the path dependence effects are compatible with a geo-historical interpretation of urban diversity.