In the ‘century of the city’, with an increasing share of the world population living in urban agglomerations, cities have not only increased in number and size, but have also turned into complex and multi-faceted organisms. A modern city is no longer a simple settlement system with buildings, infrastructure and people, but displays nonlinear evolutionary structures and trajectories as a result of an underlying complex force field comprising a multiplicity of (internal and external) actors and of (tangible and immaterial) constituents that altogether shape contemporaneous city life. Urbanity has become a “modus vivendi” of the 21st century, in which urban agglomerations are the geographic projections of an emerging new society characterized by connectivity, mobility and flexibility. The complexity of ever growing cities in this world prompts serious policy concerns regarding environmental quality, energy use, transport accessibility, social cohesion, labour and housing markets, public amenities, safety, effective governance, local well-being, and so forth. To cope with all these challenges – and many more – in the harsh reality of urban policy and management, city authorities have over the years often resorted to sectoral responses, without sufficient regard of the interwovenness of a complex urban system and without carefully basing necessary urban decisions and adjustments on a solid and verified information base that maps out the multidimensional complexity of the urban area concerned. Consequently, urban policy tends to become fragmented and not supported by quantitative accountability and solid test principles.