Abstract: Low-income, nonresident fathers owe a disproportionate amount of child support arrears, creating potential challenges for these fathers and their family relationships. This article uses mediation analysis to provide new evidence about how and why child support debt is related to paternal involvement using information from 1,017 nonresident fathers in the Fragile Families Study. Results show that child support arrears are associated with nonresident fathers having significantly less contact with children, being less engaged with them in daily activities, and providing less frequent in-kind support 9 years after the birth. This negative association between child support debt and father involvement is most strongly and consistently mediated by the quality of the relationship between the biological parents. Although child support policies are designed to facilitate fathers' economic and emotional support, these results suggest that the accruement of child support debt may serve as an important barrier to father involvement.