We use data from three waves of the Fragile Families Study (N = 2,111) to examine the prevalence and effects of mothers' partnership changes between birth and age 3 on children's behavior. We find that children born to unmarried and minority parents experience significantly more partnership changes than children born to parents who are married or White. Each transition is associated with a modest increase in behavioral problems, but a significant number of children experience 3 or more transitions. The association between instability and behavior is mediated by maternal stress and lower quality mothering. The findings imply that policies aimed at reducing maternal stress and partnership instability may improve child well-being.