As the number of people in cities around the world continues to increases, so does the risk of issues like overcrowding, waste management and resources.
More than half of the world's population was living in urban settlements in 2016, according to the United Nations' The World's cities data booklet. The booklet defines a city using the term 'urban agglomeration,' which considers the extent of the contiguous urban area, or built-up area, to determine the city's boundaries.
As of 2016, the world had 31 megacities -- cities with 10 million inhabitants or more -- and they are growing, in both size and number.
According to the U.N., the world is expected to add 10 new megacities by 2030. Tokyo is expected to remain the world's largest city in 2030, despite a projected decline of nearly 1 million inhabitants.
Check out the world's 31 megacities: