Escalating rates of early retirement are imposing fiscal pressure on retirement systems around the world. In some developed countries, the labor-force participation rates of men ages 60-64 have fallen by 75 percent over the last three decades. One explanation for this striking decline is social security program provisions which create disincentives to continued labor-force participation by older workers. There are substantial differences among developed nations in the labor-force participation of older workers. While two-thirds of 60-year-old American males are working, only one-quarter of men that age are working in Belgium. Over the entire 55-65 age range, 63 percent of American males are working, compared with only 40 percent of French males and 33 percent of Belgians males. There is strong evidence that the early retirement provisions of social security systems in developed countries determine the modal age of retirement. There is a strong relationship between early retirement ages and labor-force withdrawal rates; for example, in France, 60 percent of those working at the early entitlement age of 60 leave the labor force at that age. The core of this analysis is the construction of "implicit tax/subsidy rates" on additional work at older ages through each nation's social security system. These rates measure the change in a worker's retirement wealth entitlement from delaying retirement for one year, relative to the amount that would have been earned over that year. The U.S. Social Security system has an actuarial adjustment for delayed benefits claiming and other features that avoid financial incentives to leave the labor force at age 62 for a married worker, there is a slight disincentive to work for single workers and high wage earners. However, at ages 65 and older there is a stronger incentive to leave the labor force, with implicit tax rates on work of 19 percent for married workers and 33 percent for single workers. By comparison, other nations do not have actuarially fair adjustments, and as a result impose substantial taxes on additional work at older ages. In several countries, implicit tax rates on work at older ages approach or exceed 100 percent. This is because by delaying retirement, workers forgo benefits which often replace close to their full wage, in addition to having to pay the high payroll taxes required to finance generous social security benefits. There is a striking correlation across nations between high implicit tax rates on additional work and low labor-force participation rates among older workers. This suggests that social security program incentives are an important determinant of retirement. These findings have important policy implications for reforming social security programs in the United States and abroad. Policymakers must consider how program reforms will affect incentives for continued work at older ages.