What is a city? Definitions vary, but it is generally agreed that, at minimum, it’s a permanent human settlement — something more than a nomadic encampment. Most would probably add that it must be sizable — though what that means would depend on the era — and be governed by laws. Some authorities maintain that it must also have a functioning sanitation system. It goes on from there.

Archeologists dispute the identity of the first agglomeration that could be called a city, but it was most probably in Mesopotamia, today’s Middle East. The strongest contender for the honor is the long-vanished Eridu, in modern-day Iraq, whose ruins were first excavated in 1855. The oldest continuously inhabited city, on the other hand, is most likely Damascus, which may date back as much as 11,000 years — though another Syrian city, Aleppo, also has a claim to the honor, as does Jericho, in the Palestinian Territories.

Jericho was said to have been the world’s largest city back around 7000 B.C. It had about 2,000 inhabitants at the time. By 500 A.D., Constantinople — modern-day Istanbul — was home to about half a million people (it’s now the fourth-largest city on the planet). By 900, another city on today’s largest list, Baghdad, had a population of about a million. Three hundred years later, the comparatively little-known Chinese city of Kaifeng surpassed a million. Today, 17 of the world’s 50 most populous cities are in China.

Some 33 of them are in Asia, in fact. A mere seven are in the Americas, along with four in Europe, three in the Middle East, and three in Africa.

Why do some cities grow more than others? The obvious physical reasons: liveable climate, abundant natural resources, accessibility (it’s no accident that most fast-growing cities in the pre-airplane age were situated on rivers or seacoasts), and room to expand geographically.

Social and political factors apply, too. The “bright lights, big city” factor has always drawn rural populations to town, for instance. So have employment opportunities, real or imagined — the real ones often spurred by government investment or tax incentives to help grow industry.

As the world’s population increases, inexorably — by about 83 million each year — so does the population of most cities. Some grow so rapidly that they leap-frog up the ladder, overshadowing municipalities that had been among the most populous. This list reflects where things stand today; in a year or two or five, the order might be different, and some new large cities might have joined the roster.

Click here to see the 50 largest cities in the world.
Click here to see the 25 tiny cities around the world.

24/7 Wall St. reviewed the largest cities in the world as ranked in March 2018 by The City Mayors Foundation, an international research think tank dedicated to urban affairs. The foundation’s research was conducted during 2017 and early 2018. The City Mayors estimates are based on various national and international sources. Country population figures are for 2018 and were obtained from the U.S. Census Bureau’s International Data Base (IDB).