Cities are home to half the world’s seven billion people. Current urbanization trends indicate that an additional three billion people will be living in cities by 2050, increasing the urban share of the world’s population to two-thirds. Cities face major challenges—extreme urban poverty, poor living conditions for one billion slum dwellers, constraints on productivity due to lack of basic infrastructure, and risks due to natural disasters and climate change. However, they also have an extraordinary potential for transformational change due to their: concentration of economic activity, potential for social transformation, high levels of annual investment in infrastructure and buildings, high degree of innovation, nimble local governments, connection to surrounding rural and natural environments, ability to reduce eco-footprints by densification, and suitability for systems-based solutions. A Sustainable Development Goal (SDG) on cities could maximize the potential for the dynamics of global urbanization to be directed towards sustainable development solutions for pressing urban problems.