Everyday for the next 35 years, an average of 175,000 people will move to or be born in cities in the developing world - mostly in fast growing areas like Asia and Africa.
But did you know cities can be good for the planet too? Their compact nature means that water, power, building materials, transportation and land can be used much more efficiently.
In cities, as in life, we have a choice: sprawl or growl tall?
The crew at MinuteEarth investigate the best way to make a city more sustainable.