In Illinois, Parentage actions are cases filed between two parties who have a child together, but are not married, in order to either (1) establish the existence of a parent-child relationship (i.e., determine that someone is the biological mother or father of a child), or (2) declare the non-existence of a parent-child relationship (i.e., determine that someone is not the biological mother or father of a child). On the contrary, divorce actions are field between a married couple who may or may not have children. Although the purpose of purpose of parentage cases is to either establish or disestablish a parent-child relationship, most parentage cases are filed in order to obtain child support from one parent to the other, or to allocate parental decision making responsibilities and parenting time.

The Illinois Parentage Act of 2015 provides the Court with the authority to establish parentage, allocate decision making responsibilities, allocate parenting time, and award child support in parentage cases. Under the Act, a parent-child relationship between a woman and a child is established by (1) the woman having given birth to the child, (2) an adjudication by the court declaring the woman's parentage, (3) adoption of the child by the woman, or (4) a valid gestational surrogacy arrangement that complies with the Gestational Surrogacy Act in Illinois. A parent-child relationship between a man and a child is established (1) if the man signed a voluntary acknowledgment of paternity; (2) an adjudication of the man's parentage by the court; (3) adoption of the child by the man; or (4) a valid gestational surrogacy arrangement that complies with the Gestational Surrogacy Act in Illinois.

A person is presumed to be the parent of a child if (1) the parents have entered into a marriage or civil union, and the child is born to the mother during the marriage; (2) the parents entered into a marriage or civil union and the child is born within 300 days after the marriage or civil union is terminated for any reason (such as death of a party, divorce, or legal separation);  (3) before the birth of the child, the parents entered into a marriage or civil union in apparent compliance with the law, even if the attempted marriage or civil union is later declared invalid, and the child is born during the invalid marriage or within 300 days after the termination of the legal relationship; (4) after the child's birth, the parents enter into a marriage or civil union, even if the marriage or civil union could be declared invalid, and the parent is named as the child's parent on the birth certificate with the person's written consent.

Under the Illinois Parentage Act of 2015, only certain people have standing  to file a parentage action: the child, the mother of the child, a pregnant woman, a man presumed or alleging himself to be the parent of a child, a woman presumed or alleging herself to be the parent of the child, the support-enforcement agency or other governmental agency authorized by law, any person or public agency that has physical possession of, has custody of, has been allocated parental responsibilities for, is providing financial support to, or has provided financial support to the child, the Department of Healthcare and Family Services if it is or has provided financial support to the child or is assisting in child support enforcement, and an authorized adoption agency.