Multiple federal programs provide homelessness assistance through programs targeted to those experiencing homelessness or through mainstream programs that broadly assist low-income populations. Programs' definitions of homelessness range from including primarily people in homeless shelters or on the street to also including those living with others because of economic hardship. GAO was asked to address (1) the availability, completeness, and usefulness of federal data on homelessness, (2) the extent to which research identifies factors associated with experiencing homelessness, and (3) how differences in definitions and other factors impact the effectiveness of programs serving those experiencing homelessness. GAO reviewed laws, agency regulations, performance and planning documents, and data as well as literature on homelessness, and spoke with stakeholders, such as government officials and service providers, about potential barriers. GAO recommends that Education, HHS, and HUD (1) develop a common vocabulary for homelessness; and (2) determine if the benefits of collecting data on housing status in targeted and mainstream programs would exceed the costs. To the extent that the agencies explicitly addressed the recommendations in their comments, they agreed with them. Appended are: (1) Objectives, Scope and Methodology; (2) Bibliography of Studies GAO Reviewed That Analyze Factors Associated with Homelessness; (3) Comments from the Department of Education; (4) Comments from the Department of Health and Human Services; (5) Comments from the Department of Housing and Urban Development; (6) Comments from the Executive Director of the U. S. Interagency Council on Homelessness; and (7) GAO Contact and Staff Acknowledgments. (Contains 3 tables, 2 figures and 48 footnotes.)