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 California, paternity, or legal fatherhood, is crucial for child custody and support matters. If a child is born to a married couple, the husband is presumed to be the child's legal father. For children born to unmarried parents, the biological father does not have legal rights until paternity is established. Paternity can be established either voluntarily by signing a Voluntary Declaration of Paternity (AOP) or through a court order, which may involve a paternity test. If a man has reason to believe he is not the biological father of a child for whom he has been identified as the father, he can challenge paternity by filing a petition with the court. The court may terminate his parental rights and obligations, such as child support, if genetic testing proves he is not the biological father. To add a father's name to a birth certificate in California, paternity must be established either voluntarily or through a court order.