Objective: To explore potential mediating and moderating factors that influence the association abstract between paternal depression in the postnatal period and subsequent child behavioral and emotional problems. Methods: A population-based cohort (N = 13 822) from the Avon Longitudinal Study of Parents and Children (ALSPAC) was recruited during pregnancy. Paternal and maternal depressive symptoms were assessed with the Edinburgh Postnatal Depression Scale at 8 weeks after the birth of the child. Child outcomes were assessed at 3.5 years by using the Rutter revised preschool scales and at 7 years by using the Strengths and Difficulties Questionnaire. Path analysis was used to assess hypothesized mediators (ie, depression in the other parent, couple conflict, and paternal noninvolvement) of the associations between both paternal and maternal depression and child outcomes. We also tested for hypothesized moderators (ie, paternal education and antisocial traits). Results: Family factors (maternal depression and couple conflict) mediated two-thirds of the overall association between paternal depression and child outcomes at 3.5 years. Similar findings were seen when children were 7 years old. In contrast, family factors mediated less than one-quarter of the association between maternal depression and child outcomes. There was no evidence of moderating effects of either parental education or antisocial traits. Conclusions: The majority of the association between depression in fathers postnatally and subsequent child behavior is explained by the mediating role of family environment, whereas the association between depression in mothers and child outcomes appears to be better explained by other factors, perhaps including direct mother-infant interaction.