Published in June 2004, this updated study presents more-detailed 100-year projections for Social Security under current law, as a prelude to evaluating any legislation affecting that program. The report focuses on the resource demands of the Social Security system, the program’s finances, and projections of the benefits received by individuals in different age and income groups. In keeping with CBO’s mandate to provide objective analysis, the study makes no recommendations. (Background on Social Security, including information about the program’s structure and financing, underlying demographic trends, and strategies that have been proposed to prepare for the aging of the U.S. population, can be found in an earlier CBO publication, Social Security: A Primer, published in September 2001) The uncertainty about Social Security that individuals and policymakers face is an important economic and policy consideration. To display the uncertainty inherent in long-term projections, CBO calculates not only basic projections for Social Security but also ranges of possible outcomes. To do that, CBO uses standard statistical techniques to analyze patterns of past variation in most of the demographic and economic factors that underlie the analysis, such as fertility and mortality rates, interest rates, and the rate of earnings growth. It then uses its model to run hundreds of projections, each time with random variations in the assumed values for those factors that are equivalent to the variation observed historically.