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). When paternity is established by one of these methods and the father is not the primary custodian or caretaker of the child, he will be ordered to pay child support.

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 Washington State, paternity, or legal fatherhood, is crucial for child custody and support matters. A husband is typically presumed to be the father of a child born to his wife during their marriage. For children of unmarried parents, the biological father has no legal rights until paternity is established. Paternity can be confirmed either by a court-ordered DNA test or by the father voluntarily signing an Acknowledgment of Paternity (AOP). Once paternity is established, if the father is not the primary caregiver, he may be required to pay child support. If a man has reason to believe he is not the biological father, he can file a petition to challenge paternity. The court may terminate his parental rights and child support obligations if genetic testing proves he is not the father. Additionally, establishing paternity is linked to the process of adding the father's name to the child's birth certificate.