Using longitudinal data from the Maternal and Child Supplement of the National Longitudinal Survey of Youth (N = 3,862) and Hierarchical Linear Models, we estimated associations of family structure states and transitions with children's achievement and behavior trajectories during middle childhood. Results suggest that residing in a single-mother family was associated with small increases in behavior problems and decreases in achievement and that residing in a social-father family was associated with small increases in behavior problems. Family structure transitions, in general, were associated with increases in behavior problems and marginally associated with decreases in achievement. Transitioning to a single-mother family was associated with increases in behavior problems, whereas transitioning to a social-father family was associated with decreases in reading achievement.