Student mobility-students making nonpromotional school changes-is widespread in many schools and districts throughout the United States. Mobility not only can harm the students who change schools, it can also harm the classrooms and schools they attend. This article examines the incidence, causes, and consequences of student mobility in the United States. Research reveals that the causes and consequences of mobility are more complicated than many people assume. The final part of the article discusses what parents, schools, districts, and policymakers can do to address this growing educational problem. [S]ince I've been here it's well over 800 students from July through December who have left. They have been replaced by 800 more coming in. We're about 3800 as far as active students right now . . . so you're constantly dealing with 1000 students leaving, 1000 students coming in. (Principal from California high school, as quoted in Rumberger, Larson, Ream, & Palardy, 1999, p. 29) Student mobility is widespread in many schools and districts through the United States. Student mobility is the practice of students making nonpromotional school changes, often during the school year. Mobility not only can harm the students who change schools, it can also harm the classrooms and schools they attend. Many educators believe that student mobility is an inevitable result of students changing residences. Indeed, the majority of student mobility in the United States is a result of families changing residences. But not all student mobility is the result of residential mobility, particularly at the high school level. Schools contribute to mobility due to such things as overcrowding, class size reduction, suspension and expulsion policies, school choice, and the general academic and social climate. Schools can also help address the problem by both reducing unnecessary mobility and mitigating its harmful effects. This article examines the issue of student mobility. It first examines data on the incidence of student mobility in the United States. It then reviews the research literature on the social and academic consequences of mobility for both students and schools. Next, it reviews the research literature on the causes of mobility. Finally, it discusses what parents, schools, districts, and policymakers can do to address this growing educational problem. THE INCIDENCE OF STUDENT MOBILITY Student mobility is pervasive in the United States. According to data collected through the National Assessment of Educational Progress (NAEP) 1998 Math Assessment, 34% of 4th graders, 21% of 8th graders, and 10% of 12th graders changed schools at least once in the previous two years (see Figure 1). The incidence of student mobility varies by race, ethnicity, and family income. For example, data from the 1998 NAEP show that: (a) 41% of Hispanic American and 45% of Black fourth-grade students changed schools in the last two years, compared to 27% of White and 33% of Asian American fourth-grade students; and (b) 43% of fourth-grade students who were eligible for the national school lunch program (i.e., low-income students) changed schools in the last two years, compared to 26% who were not eligible (U.S. Department of Education, 2002). The incidence of student mobility is even higher when viewed over a student's entire elementary and secondary career. Based on data from a national longitudinal study of a cohort of eighth graders in the United States, more students made nonpromotional school changes during their elementary and secondary school careers than remained in a stable pattern of attending a single elementary, middle, and high school (Rumberger et al., 1999, p. 23). School changes were more common during elementary school than during secondary school. In fact, mobility is the norm during elementary school, while it is the exception during high school. Student mobility not only varies widely among students, but also among schools. …