Despite varying definitions, it is without a doubt that city populations have been on the rise steadily throughout the 20th and 21st centuries. The number of people living in urban areas with populations of 10 million or more has jumped from 69.5 million in 1975 to a staggering 472.8 million in 2015. China has had a significant impact on this growth, with more than half of its population living in urban areas in 2017. As of 2018, the three largest urban agglomerations are Tokyo, Delhi, and Shanghai with populations of 37.4, 28.5, and 25.6 million people, respectively.
Megacities also present various economic opportunities for countries and its citizens, including possibilities for innovation, education, interconnectivity and development. It is estimated that by 2030, 5 percent of the world's top 20 megacities will contribute 21 percent of global GDP. New York City alone is projected to have a GDP of approximately 44 billion U.S. dollars by 2030. Accompanying these enormous productive boosts are various challenges for increasingly concentrated urban areas, namely cities marked by slums, high crime rates, homelessness, traffic congestion, and troubles managing energy and waste resources. These issues have and continue to pose complex challenges for countries rapidly urbanizing in the 21st century.