The world is getting older and no one knows exactly what life will be like in tomorrow's older societies. But we do know that age dependency ratios—the ratio of retirees to workers—will be much higher than we see today. The combined effects of fewer workers, more retirees and longer retirement periods threaten not only the sustainability of pension systems but also the broader economic prospects of many developed countries. This analysis describes current trends in birth rates, longevity and labor force participation and productivity, the cross-border flow of capital, the globalization of labor markets, the financial viability of social insurance programs, and the ways economic output is shared between working-age and retiree populations.