Yet, these often cramped, unplanned and unsafe structures are home to millions of the planet's poorest people.
Take, Heliopolis in Sao Paulo, Brazil, ifor example. It has existed for more than 40 years and now has a a population of more than 100,000 - making it one of of the biggest slums in the Americas.
Now, the town's planners are renergising and rethinking this dynamic community.
Adam Shaw meets one of Brazil's top architects who was challenged to make this corner of Brazil a better place to live.
He responded by building radical new structures in a socially responsible and profitable way.
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