Educational achievement of students can be negatively affected by their changing schools often. The recent economic downturn, with foreclosures and homelessness, may be increasing student mobility. To inform Elementary and Secondary Education Act of 1965 (ESEA) reauthorization, the US Government Accountability Office (GAO) was asked: (1) What are the numbers and characteristics of students who change schools, and what are the reasons students change schools? (2) What is known about the effects of mobility on student outcomes, including academic achievement, behavior, and other outcomes? (3) What challenges does student mobility present for schools in meeting the educational needs of students who change schools? (4) What key federal programs are schools using to address the needs of mobile students? GAO analyzed federal survey data, interviewed U.S. Department of Education (Education) officials, conducted site visits at eight schools in six school districts, and reviewed federal laws and existing research. Appended are: (1) Scope and Methodology; (2) Data on Characteristics of Mobile Student Populations; (3) Data on Characteristics of Schools Regarding Mobile Student Populations; (4) Literature Review of Published Research on Student Mobility; and (5) GAO Contact and Staff Acknowledgments. (Contains 19 tables, 8 figures, and 24 footnotes.)