Many federal assistance programs are funded by formula grants that have historically relied at least in part on population data from the decennial census and related data to allocate funds. In June 2009, the Census Bureau reported that in fiscal year 2007 the federal government obligated over $446 billion through funding formulas that rely at least in part on census and related data. Funding for federal assistance programs continues to increase. Government Accountability Office (GAO) was asked to determine (1) how much the federal government obligates to the largest federal assistance programs based on the decennial census and related data, and how the Recovery Act changed that amount; and (2) what factors could affect the role of population in grant funding formulas. To answer these objectives, GAO identified the 10 largest federal assistance programs in each of the fiscal years 2008 and 2009 based on data from the President's fiscal year 2010 budget. GAO reviewed statutes, agency reports, and other sources to obtain illustrative examples of how different factors could affect the role of population data in grant funding.