As a strong urbanization trend is consolidating worldwide, cities – “global” cities in particular – are playing an increasingly important role in the economic, infrastructural, and social development of countries and regions. This trend, however, also brings with it new problems: from the social inclusion and mobility of a growing number of citizens, to the environmental impact of cities that face the challenges posed by a rapid, and at times chaotic urbanization. In order to overcome these challenges, the vision of cities that are “smart” emerged over the last decades. But is a “Smart City” also a sustainable one? What should city planners do to foster an urban development that is really sustainable? And what role should citizens and communities play to enhance a bottom-up transition towards urban sustainability? What are the most innovative and interesting examples worldwide?