For the first time in history, half of the world's population lives in urban areas and it is expected that, by 2050, that figure will rise to above two-thirds. A large proportion of this urban growth will be taking place in the cities of the developing world, where the provision of adequate health, shelter, water and sanitation and climate change adaptation efforts for rapidly-growing urban populations will be an urgent priority. This transition to an urban world could be a negative transformation; but, if well-planned, it could also offer an unprecedented opportunity to improve the lives of...