The UN says that the world will have undergone the largest and fastest period of urban expansion in human history by 2050 as two-thirds of the world's population will then live in cities.
In the 1990s, ten cities around the world were considered 'megacities' with a population of more than 10 million. Since then, the number of such cities has risen to 21.
The shift to a largely urban global population means the work of city planners is changing.
TRT World’s Tadhg Enright spoke to an urban planner in London, one of the world’s best-known megacities.