In the United States today, we have two legal bases for parentage, biology and function. Biological parenthood is usually controlling when the issue is liability for child support, and functioning as a parent is considered, if at all, only when the primary issue is custody or access to a child. These two strands of parentage law derive from what Jacobus tenBroek called the dual system of family law. While the divided law that ten Broek describes is centuries old, until fairly recently, the two strands ran in parallel and did not have much impact on each other. However, in the last several decades they have evolved and, as a result, are today on a collision course when the identity of a child’s legal parents must be determined. Child support law has come to be predominantly welfare-driven; in tenBroek’s terminology, it has taken on characteristics of “public law,” regardless of whether it applies to the poor or to the upper classes.The law that governs private disputes over custody, visitation and the like continues to have the characteristics of “private law.” The difference in these approaches is especially apparent in the law of parentage. If child support is the ultimate question, parentage will likely be determined according to biology, the principle favored by the “public law approach.” If custody or access is the main issue, private law principles, which tend to respect functional parenthood, are more likely to be invoked. And yet, once legal parentage is determined, it applies to determine the rights and duties of the involved adults vis-à-vis the child, regardless of context. The article argues that as biology-based parentage becomes more pervasive, it threatens to displace rules based on functional parent-child relationships, which would be harmful to many children and their families. To avoid this result, I argue that we need a substantive law of parentage that recognizes the importance of biology while preserving a realm in which functional relationships are protected. To make this law politically viable, we also should reject some child support rules and practices that treat men unfairly and, in so doing, suggest that biology is the only thing that matters for determining legal parentage.