Cities already shelter more than half of humankind and appear, more than ever, as Janus-faced realities, the best and worst of places, vulnerable but still full of hope and will to advance towards sustainable development. The battle for sustainable development will be won or lost in (or because of) cities. The first reason is simply their sheer size, this extraordinary concentration of people and activities. Furthermore, cities are the only places where people and resources congregate at a point beyond which synergetic effects become more important than simply additive ones. Cities also promote open democracies and produce models that are disseminated to the World. The book offers a 360° view of the challenges facing cities in terms of growth and competitiveness, governance, sustainability, social inclusion, science, culture and the arts, urban renaissance and planning, and indicates innovative responses by pioneer cities.