Most children in the U.S. today will experience one or more changes in family structure. The present study explores the implications of this trend for child development by investigating the conditions under which family structure changes matter most to child well-being. Using data from the Maternal and Child Supplement of the National Longitudinal Survey of Youth (N = 3,492), it estimates how changes in family structure experienced during 4 different developmental periods relate to concurrent and subsequent changes in children's behavioral trajectories. We estimate associations separately for children born to married and unwed parents to determine whether family instability has different associations with children's behavior across policy-relevant family types. Results indicate that changes in family structure during the first 3 years of life predict children's behavioral development more consistently than later changes, changes into a single-parent family have different implications for children's development than changes into a blended family, and changes in family structure matter more for children born to married parents than children born to unwed parents.