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 Washington State, paternity, or legal fatherhood, is crucial for child custody and support matters. If a child is born during a marriage, the husband is typically presumed to be the father. For children born to 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). 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 file a petition to challenge paternity. The court may terminate his parental rights and responsibilities, including child support obligations, if genetic testing proves he is not the father. Additionally, establishing paternity is a necessary step for adding the father's name to the child's birth certificate.