In connection with the Omnibus Appropriations Act, 2009, GAO (Government Accountability Office) was required to study the What Works Clearinghouse (WWC), a federal source of evidence about effective education practices. Operating through a 5-year contract awarded by the U.S. Department of Education's Institute of Education Sciences (IES), the WWC reviews education research and disseminates its findings. GAO examined: (1) the extent to which the WWC review process meets accepted standards for research evaluation and how the WWC has responded to recommendations and criticism, (2) how WWC output and costs have changed over time and how its performance is measured, and (3) how WWC products are disseminated and how useful educators find them to be. To conduct its work, GAO reviewed WWC-related documents, analyzed cost and performance data, surveyed all states and a nationally representative sample of school districts, and interviewed IES officials, WWC contractors, researchers, and others. GAO recommends that IES: develop and implement strategies to avoid backlogs in WWC product reviews; establish performance measures related to costs and usefulness; and improve dissemination efforts to promote awareness and use of the WWC. Education generally agreed with GAO's recommendations. Appendices include: (1) Objectives, Scope, and Methodology; (2) Other Sources of Information Districts Use To Identify Effective Education Practices; (3) IES and WWC Response to Expert Panel Recommendations; (4) Comments from the Department of Education; and (5) GAO Contact and Staff Acknowledgments. (Contains 9 tables, 15 figures, and 67 footnotes.)