As of 1975, there were three metro areas with at least 10 million people — Tokyo, New York and Mexico City. A list of the 10 largest cities at that time would have included Paris (now 25th), Moscow (22nd) and Los Angeles (21st). Now there are 31 megacities with at least 10 million people, and most of them are in the developing world. The UN projects 10 more will join the list by 2030, and all but one (Bogotá) is in Africa or Asia.

The Big Picture: 518 million people (7% of the global population) now live in megacities of 10 million or more people. That's a tenfold increase from four decades ago, and it's radically changing the way people live, work and view the world.

The world's largest metro areas

Tokyo, Japan. 38 million

At a glance: Japan's capital is major international financial center and has the biggest economy of any global metro area. Tokyo's restaurants have by far the most Michelin stars of any city.

Japan's capital is major international financial center and has the biggest economy of any global metro area. Tokyo's restaurants have by far the most Michelin stars of any city. GDP per capita: $43,884

$43,884 Population in 1975: 27 million (Rank: 1st)

New Delhi, India. 26 million

At a glance: India's capital has been inhabited for at least 2,500 years, and boasts multiple world heritage sites. Its growing economy draws significant foreign investment. It is also one of the world's most polluted cities.

India's capital has been inhabited for at least 2,500 years, and boasts multiple world heritage sites. Its growing economy draws significant foreign investment. It is also one of the world's most polluted cities. GDP per capita: $16,861

$16,861 Population in 1975: 4.4 million (Rank: Outside top 10)

Shanghai, China. 26 million

At a glance: Shanghai is the world's biggest shipping port and China's financial hub. As the Economist writes

Shanghai is the world's biggest shipping port and China's financial hub. As the GDP per capita: $32,684

$32,684 Population in 1975: 7.3 million (Rank: Outside top 10)

Sao Paulo, Brazil. 21 million

At a glance: A diverse, cosmopolitan city, Sao Paulo has the biggest economy of any city in Latin America, though it lacks the glamour of nearby Rio de Janeiro.

A diverse, cosmopolitan city, Sao Paulo has the biggest economy of any city in Latin America, though it lacks the glamour of nearby Rio de Janeiro. GDP per capita: $27,366

$27,366 Population in 1975: 13 million (Rank: 5th)

Mumbai, India. 21 million

At a glance: India's financial capital is also the home of the Bollywood film industry. 41 billionaires live in Mumbai, but more than half of the population resides in slums.

India's financial capital is also the home of the Bollywood film industry. 41 billionaires live in Mumbai, but more than half of the population resides in slums. GDP per capita: $10,147

$10,147 Population in 1975: 7.1 million (Rank: Outside top 10)

Mexico City, Mexico. 21 million

At a glance: Mexico's sprawling capital city was first settled by the Aztecs. It's the center of Mexico's politics and economy.

Mexico's sprawling capital city was first settled by the Aztecs. It's the center of Mexico's politics and economy. GDP per capita: $23,017

$23,017 Population in 1975: 11 million (Rank: 3rd)

Beijing, China. 20 million

At a glance: China's capital city is home to many cultural landmarks, including the Forbidden City and Temple of Heaven. It's home to 52 Fortune Global 500 companies, the most of any city. Air pollution is a significant concern.

China's capital city is home to many cultural landmarks, including the Forbidden City and Temple of Heaven. It's home to 52 Fortune Global 500 companies, the most of any city. Air pollution is a significant concern. GDP per capita: $30,335

$30,335 Population in 1975: 6 million (Rank: Outside top 10)

Osaka, Japan. 20 million

At a glance: Historically the center of Japanese cuisine and commerce, Osaka is less flashy than Tokyo but has one of the largest economies of any city in the world.

Historically the center of Japanese cuisine and commerce, Osaka is less flashy than Tokyo but has one of the largest economies of any city in the world. GDP per capita: $36,335

$36,335 Population in 1975: 10 million (Rank: 4th)

Cairo, Egypt. 19 million

At a glance: An ancient city with some of the world's most impressive Islamic architecture, Egypt's capital also has a bustling metro system. It was the site of the Tahrir Square protests in 2011.

An ancient city with some of the world's most impressive Islamic architecture, Egypt's capital also has a bustling metro system. It was the site of the Tahrir Square protests in 2011. GDP per capita: $7,843

$7,843 Population in 1975: 6 million (Rank: Outside top 10)

New York, USA. 19 million

At a glance: A global center of finance and the arts, New York has been the largest U.S. city since the country's first census

A global center of finance and the arts, New York has been the largest U.S. city since the country's GDP per capita: $74,000

$74,000 Population in 1975: 16 million (Rank: 2nd)

Key trends

Of the world's 31 megacities, 6 are in China and 5 are in India. By 2030, both countries will have 7 megacities.

There will be 8 cities with ~25 million or more people by 2030 — we may soon need a new definition for what qualifies as a megacity.

Six of the world's 10 largest cities are in Asia. Zero are in Europe.

Worth noting: Estimates of urban populations vary widely, mainly because the boundaries and definitions used can be subjective. All of the population data cited above comes from the United Nations.