There are currently 1.5 billion people aged 12-24 in the world, with 1.3 billion of them in developing countries. Many developing countries have reached or will soon reach a historical peak in the size of their youth population, a peak that is probably the largest number of young people these countries will ever see. This short article examines this “youth bulge,” charting changes in the 12-24 year-old population in major areas of the world since 1950 and projected to 2050; identifying causes and effects of these changes; and looking ahead at economic challenges and opportunities.