We live in a world of cities. Since 2008, the majority of the population has been concentrated in large metropolitan areas. Urbanisation is often mentioned as one of the main drivers of economic, social, political and environmental transformation. As this trend continues to rise, challenges and opportunities that will define our future are likely to arise in cities. Cities are like a living organism, with the ability to organically absorb thousands of inhabitants day by day. They are able to create massive amounts of economic abundance: urban affluence is responsible for more than 75% of global wealth and yet inequality in cities is larger than at the national level and that in small towns. And it continues to grow. New innovations that help to tackle environmental degradation are born in urban hubs and knowledge clusters, yet large metropolises account for most of the global environmental footprint.