As a result of societal changes, fathers participate more actively in child care than they used to. In this article, we propose a context-dependent biobehavioral model of emergent fatherhood in which sociocultural, behavioral, hormonal, and neural factors develop and interact during the first 1,000 days of fatherhood. Sociocultural factors, including different expectations of fathers and varying opportunities for paternal caregiving through paid paternal leave, influence paternal involvement. Levels of hormones (e.g., testosterone, vasopressin, oxytocin, cortisol) predict fathers’ parenting behaviors, and involvement in caregiving in turn affects their hormones and brain responses to infant stimuli. The birth of the first child marks the transition to fatherhood and may be a critical period in men’s lives, with a smoother transition to fatherhood predicting more optimal involvement by fathers in subsequent years. A focus on prenatal and early postnatal fathering may pave the way for developing interventions that effectively support fathering during pregnancy and in the first years of their children’s lives.