We use investment theory to study the trajectory of father involvement over time as a function of union status and union transition. Data on 4,311 mother-father pairs are from the Fragile Families and Child Well-Being Study. Fathers who remain in a continuous co-residential union, who transition from cohabitation to marriage, or who transition from a non-coresidential state to a co-residential union experience the highest levels of father involvement. Fathers who are continuously nonresident exhibit the lowest levels of involvement . Cohabiting fathers exhibit higher levels of father involvement than married fathers. A positive coparenting relationship is associated with higher levels of involvement . We discuss the place of cohabiting families in light of our findings.