This paper examines the hypothesis that high levels of residential mobility among nonintact families account for part of the well-known association between living in a nonintact family and dropping out of high school. Children from single-parent families and stepfamilies are more likely than children from two-parent families to move during the school year. As much as 30% of the difference in the risk of dropping out between children from stepfamilies and children from intact families can be explained by differences in residential mobility. Previously, mechanisms explaining school failure on the part of children in nonintact families were more plausible for children in single-parent families than for children in stepfamilies; high levels of residential mobility apply to both groups of children. In addition, residential mobility lends itself to manipulation by public policy, with potentially remedial effects for vulnerable children.