Where paternity of the child is in question, any party with an interest may ask the court to determine paternity, of one or several potential fathers (called putative father) based initially upon sworn statements and then upon testimony, including but not limited to:

• Sexual relations by the mother with a man other than the putative father, with a description of the circumstances of the child's conception
• Lack of access to the mother by the putative father during the same time period
• Scientific proof (blood group, HLA marker, DNA testing, etc.) showing the likelihood of putative father being genetically related
• Other admissible evidence that may tend to establish paternity such as statements against interest made by the putative father

Generally, hospital records and birth certificate are not sufficient to offset this presumption of lawful paternity. A successful application to the court results in an Order of Filiation (not all states refer to it as "Filiation"), and assigning paternity to a specific man. The biological father now must support the child and may also generally speaking have visitation with the child. In a marriage where the biological father is unknown or without financial means, the court may find the husband responsible for support of the child.

In some jurisdictions the courts have also declared the male who acts as the child's father to be the father through the equitable operation of an estoppel. Once the child has been declared to be a son or daughter and has lived with the man for a period of time, the court may cloak the putative father with all of the obligations of parenthood letting the biological father, "off the hook" so to speak.

A child born to the husband and wife prior to the marriage usually becomes a child of the marriage when the marriage is performed. This was the old rule from canon law, but at common law retroactive legitimacy was not conferred on a child by the marriage of the parents after the child was conceived; a matter of some import when inheritance was at issue. The different rules have largely been abrogated in the United States by a series of judicial decrees or legislation curtailing or diminishing the disabilities of bastardy.

Paternity laws are seen by some to have defects. For example, it is claimed that when men are lied to about contraception (or even raped), certain paternity laws give men no protection.