We live in the age of the megacity.
For the first time in human history more people now live in urban settlements than in rural ones, and those urban locales are growing ever bigger. Every year, Lagos, Africa's biggest city, grows by the population of Boston.
Within 10 years, about two-thirds of the world's gross domestic product will be generated by cities, up from about half now.
Most of that growth will come from the emerging world, especially China and India, meaning the stakes to get growth and development right are increasing rapidly.
The challenges range from ensuring clean streets to confronting the impact of climate change. In Rio de Janeiro, the military roams the streets to crack down on crime; in Lagos, policy makers who have often left residents to their own devices now have to introduce governance without wrecking structures developed over the years.
From mayors and local councilors to presidents and prime ministers, getting cities right is more than a local and national question – it's one for all of us.