The stepfather relationship provides a source of potential conflict in remarriage families, because the mother and partner may have different interests in the well-being of children from a prior union. Using three different theoretical perspectives—biology, sociology, and selection—this paper examines the engagement, availability, participation, and warmth of residential fathers in married biological parent, unmarried biological parent, married stepparent, and cohabiting father families. The data come from 2,531 children and their parents who were interviewed during the 1997 wave of the Child Development Supplement to the Panel Study of Income Dynamics. Biology explains less of father involvement than anticipated once differences between fathers are controlled. Marriage continues to differentiate paternal investment levels, as do age of child and financial responsibility to nonresidential children.