Establishing paternity is the process of determining the legal father of a child. When parents are married, paternity is automatically established in most cases. If parents are unmarried, paternity establishment is not automatic and the process should be started by both parents as soon as possible for the benefit of the child. Unmarried parents can establish paternity (legal fatherhood) by signing the voluntary Declaration of Paternity. This can be done in the hospital after the child is born. A Declaration of Paternity may also be signed by parents after they leave the hospital.

Unmarried parents who sign the Declaration of Paternity form help their child(ren) gain the same rights and privileges of a child born within a marriage. Some of those rights include: financial support from both parents, access to important family medical records, access to the non-custodial parent's medical benefits, and the emotional benefit of knowing who both parents are.

In an effort to create a legal link between unmarried fathers and their children, the California Department of Child Support Services joined other states in a partnership with licensed hospitals and clinics with birthing facilities to establish the Paternity Opportunity Program (POP). This voluntary in-hospital paternity acknowledgment program, implemented in January 1995, involves about 330 of California's licensed hospitals and clinics with birthing facilities. The program has since been expanded to prenatal clinics, county welfare offices, local vital records offices, and courts.