An accurate decennial census relies on finding and counting people-- only once--in their usual place of residence, and collecting complete and correct information on them. This is a daunting task as the nation's population is growing steadily larger, more diverse, and according to the U.S. Census Bureau (Bureau), increasingly difficult to find and reluctant to participate in the census. Historically, undercounts have plagued the census and the differential impact on various subpopulations such as minorities and children is particularly problematic. GAO was asked to describe (1) key activities the Bureau plans to use to help reduce the differential undercount and improve participation, (2) the various challenges and opportunities that might affect the Bureau's ability to improve coverage in 2010, and (3) how different population estimates can impact the allocation of federal grant funds. This testimony is based primarily on GAO's issued work in which it evaluated the performance of various Census Bureau operations. The Bureau's strategy for reducing the undercount and improving participation in the 2010 enumeration appears to be comprehensive, integrated, and shaped by the Bureau's experience in the 2000 Census. If implemented as planned, the various activities the Bureau is developing should position the agency to address the undercount. Key operations include building a complete and accurate address list, implementing an Integrated Communications Campaign to increase awareness and encourage participation, and fielding special enumeration programs targeted toward historically undercounted populations. For example, the Bureau develops its address list and maps over the course of a decade using a series of operations that sometimes overlap to ensure all housing units are included. Among other activities, temporary census workers go door to door across the country in an operation called address canvassing to verify addresses. To help find hidden housing units, the Bureau's workers look for clues such as two mailboxes or utility meters that could indicate additional households. Likewise, the Bureau's communications campaign includes paid media, public relations, and partnerships with national and grassroots organizations, among other efforts, some of which will be targeted toward hard-to-count groups. Despite the Bureau's ambitious plans, a number of challenges and uncertainties remain. For example, the performance of the handheld computers that is critical to address canvassing has technical shortcomings, while the communications campaign faces the historical challenge of converting awareness of the census to an actual response. Further, success will depend in large part on the extent to which the various operations (1) start and finish on schedule, (2) are implemented in the proper sequence, (3) are adequately tested and refined, and (4) receive appropriate staffing and funding. It will also be important for the Bureau to have a real-time monitoring capability to track the progress of the enumeration, target its resources to where they are most needed, and to quickly respond to various contingencies that could jeopardize the accuracy or cost of the count. Our past work indicates that the accuracy of state and local population estimates may have an effect, though modest, on the allocation of grant funds among the states. Many of the formulas used to allocate grant funds rely upon measures of the population, often in combination with other factors. For example, we analyzed the sensitivity of Social Services Block Grants (SSBG) to alternative population estimates, rather than the actual census. We selected SSBG for our analysis because the formula, which was based solely on population, and the resulting funding allocations were particularly sensitive to alternative population estimates. Based on our simulation of the funding formula, 27 states and the District of Columbia would have gained $4.2 million and 23 states would have lost $4.2 million of the $1.7 billion in 2004 SSBG funding.