This study evaluated the effects of school mobility on reading and math achievement in the elementary grades (kindergarten to sixth grade) using meta-analysis for studies dated between 1975 and 1994. Mobility was defined as any change in schools. The sample sizes of the 26 studies examined ranged from 62 to 15 000. The statistics were converted into the effect size d. The individual effect sizes were almost all negative except in cases where the sample consisted of military personnel's dependents. The composite effect size −0.25 for reading and −0.22 for math indicated that the average achievement level of mobile students exceeded that of only 40% of the non-mobile students. This is equivalent to a 3–4 month performance disadvantage in achievement. The major predictors of variation in effect sizes were frequency of mobility, socioeconomic status, and grade. Implications for practice and research are highlighted.