Prior research has linked marital conflict to children’s internalizing/externalizing disorders, insecure attachment, and poor emotional regulation (e.g., Cummings & Davies, 2010; Cummings, Iannotti, & Zahn-Waxler, 1985). Although investigators have examined the impact of marital discord on older children (e.g., Crockenberg & Langrock, 2001), few have explored direct links in infancy (e.g., Cowan & Cowan, 1999). This study extends earlier work by examining linkages between marital functioning (conflict and harmony) and infants’ cardiac vagal tone and developmental status across 2 time points using a cross-lag approach. Differential findings were found for boys and girls, with concurrent linkages between marital love and vagal tone at 6 months for boys and girls but only for boys at 12 months. In addition, marital conflict at 6 months predicted lower cardiac vagal tone in girls at 12 months but not boys. Finally, infants’ developmental status at 6 months was found to predict marital conflict at 12 months. Higher scores on the Psychomotor Development Index (PDI) predicted greater marital conflict whereas higher scores on the Mental Development Index (MDI) predicted lower conflict. These findings are discussed in the context of the emotional security hypothesis and the spillover framework as well as differential susceptibilities to early developmental contexts.