There are more than 25 cities in the world today with populations of over 10 million, according to a recent study by the United Nations Population Division, World Economic Forum. Here, we present the top 10, ranked according to total population predicted for 2025 rather than by percentage of growth. These "megacities" include densely populated surrounding metropolitan areas. For instance, the population of New York City, which ranks no. 5 on our list and is poised to reach 24 million in 2025, includes Long Island, the Mid-Hudson Valley, the Lower Hudson Valley, New Jersey's six largest cities, Connecticut's six largest cities, and five counties in northeastern Pennsylvania. And New York is the only American city to make the list.