Paternity is legal fatherhood—and establishing it is one of the most important steps in the child custody and support processes. A man is generally presumed to be the father of children born to his wife during their marriage. For unmarried parents, the biological father does not have legal rights to his child until paternity is established. Paternity may be established through a court-ordered paternity test, or by the father signing an acknowledgment of paternity (AOP).

In a case of mistaken paternity, a man who believes he is not the father of the child may file a petition (lawsuit), and the court may generally terminate the parent-child relationship and the man’s obligation to pay child support upon proof through genetic testing that he is not the father of the child.

The process of adding a father to a birth certificate is an important issue related to paternity.

In New York, paternity is the legal recognition of a man as the father of a child. When a child is born to married parents, the husband is presumed to be the child's father. For children born to unmarried parents, the biological father does not have legal rights until paternity is established. Paternity can be established voluntarily by signing an Acknowledgment of Paternity (AOP) form, which is often done at the hospital at the time of the child's birth, or later at a local birth registrar or child support office. Alternatively, paternity can be established through a court order, which may involve a paternity test if the fatherhood is contested. If a man has been declared the father but later believes he is not the biological parent, he can file a petition to challenge paternity. The court may order genetic testing and, if the man is proven not to be the biological father, the court can terminate his parental rights and responsibilities, including child support obligations. Adding a father's name to a birth certificate in New York requires the establishment of paternity and may involve submitting the AOP or a court order to the Department of Health.