This study explored reciprocal associations between paternal child-care involvement and relationship quality by following British couples from the birth of a child until he or she reached school age. It extends the literature by distinguishing between paternal engagement in absolute terms and relative to the mother and by considering relationship quality reports of mothers and fathers and family breakdown. The analysis was based on the British Millennium Cohort Study, a representative survey of children born in 2000 and 2001 and their parents (N = 5,624 couples). The author applied ordinary least squares regression analysis with lagged dependent variables and event history modeling. Fathers' relative child-care share was positively associated with mothers' relationship satisfaction, whereas fathers' absolute child-care frequency was positively related to their own perceived relationship quality for most time periods. Fathers' relative and absolute child-care contributions were positively associated with relationship stability over the preschool years. Greater perceived relationship quality of mothers, but not fathers, was associated with more frequent paternal engagement.