The past 70 years have seen the rise of megacities: cities with more than 10 million people. But some of the fastest-growing cities are built for their new population sizes and struggle with poor sanitation, high pollution, ecological threats, and crime. But cities can also foster better lives through sustainable urban development, experiments in new technologies, and efficient use of resources. By 2050, there are projected to be 47 megacities worldwide. Here are the top 10.