Data from the Fragile Families and Child Wellbeing Study (N = 4,176) are used to examine family structure transitions and maternal parenting stress. Using multilevel modeling, we found that mothers who exit coresidential relationships with biological fathers or enter coresidential relationships with nonbiological fathers reported higher levels of parenting stress than mothers in stable coresidential relationships. Mothers who enter coresidential relationships with biological fathers reported lower levels of parenting stress than mothers who remain single. Mothers' resources, especially their relationships with biological fathers, accounted for most of the associations between transitions and parenting stress, with posttransition resources being more important than pretransition resources. Mothers with high levels of education were less affected by transitions than mothers with less education.