Paternity Law, or Fatherhood Law, is the area of law focusing on establishing or disputing fatherhood. The most common reasons for paternity disputes usually include determining a potential father's rights and obligations concerning a child or establishing rights related to inheritance after a father's death.

Paternity Law primarily deals with the legal rights and obligations of fathers and their children but may also involve the inheritance rights of the parents.

Paternity Law can also concern the rights and responsibilities of the mother for matters such as child support and custody.

Traditionally, a child born to a woman during a marriage, using the standard law system, is legally presumed to be her husband's child. This presumption can be rebutted, with conclusive evidence, to show that the husband is not the father.

Conversely, when a child is born to a married woman, any man who wishes to be recognized as the child's father affirmatively can do so by filing a Voluntary Acknowledgment of Paternity (VAP) or having the child's mother filed a paternity action to pursue the claim of paternity.

Additionally, a man may be determined by a court to be the father of a child under the doctrine of estoppel.

Most states now recognize Common Law Marriages, where a couple lives together and is married per common law, though they don't have to have a marriage license.

When a child's paternity is questioned, a man may respond to the paternity action by filing a "disavowal" action. In a disavowal action, the man attempts to avoid presumptively establishing paternity and a concomitant legal obligation to provide financial support for his child.