In the past few years, the federal government has been recording the largest budget deficits since 1945, both in dollar terms and as a share of the economy. Consequently, the amount of federal debt held by the public has surged. At the end of 2008, that debt equaled 40 percent of the nation's annual economic output (gross domestic product, or GDP)--a little above the 40-year average of 38 percent. Since then, the figure has shot upward: By the end of this year, the Congressional Budget Office (CBO) projects, federal debt will exceed 70 percent of GDP--the highest percentage since shortly after World War II. The sharp rise in debt stems partly from lower tax revenues and higher federal spending caused by the severe economic downturn and from policies enacted during the past few years. However, the growing debt also reflects an imbalance between spending and revenues that predated the recession. Whether that debt will continue to grow in coming decades will be affected not only by long-term demographic and economic trends but also by policymakers' decisions about taxes and spending. The aging of the baby-boom generation portends a significant and sustained increase in the share of the population receiving benefits from Social Security and Medicare, as well as long-term care services financed by Medicaid. Moreover, per capita spending for health care is likely to continue rising faster than spending per person on other goods and services for many years (although the magnitude of that gap is uncertain). Without significant changes in government policy, those factors will boost federal outlays relative to GDP well above their average of the past several decades--a conclusion that holds under any plausible assumptions about future trends in demographics, economic conditions, and health care costs. According to CBO's projections, if current laws remained in place, spending on the major federal health care programs alone would grow from more than 5 percent of GDP today to almost 10 percent in 2037 and would continue to increase thereafter.1 Spending on Social Security is projected to rise much less sharply, from 5 percent of GDP today to more than 6 percent in 2030 and subsequent decades. Altogether, the aging of the population and the rising cost of health care would cause spending on the major health care programs and Social Security to grow from more than 10 percent of GDP today to almost 16 percent of GDP 25 years from now. That combined increase of more than 5 percentage points for such spending as a share of the economy is the federal government's programs and activities equivalent to about $850 billion today. (By comparison, spending on all of, excluding net outlays for interest, has averaged about 18.5 percent of GDP over the past 40 years.) If lawmakers continued certain policies that have been in place for a number of years or modified some provisions of current law that might be difficult to sustain for a long period, the increase in spending on health care programs and Social Security would be even larger. Absent substantial increases in federal revenues, such growth in outlays would result in greater debt burdens than the United States has ever experienced.