There is no statewide licensing law in Illinois; however, a person licensed in Illinois as a “private detective” may serve original process in all counties except for Cook County without special appointment. In order for PIs to serve in Cook County, the court upon motion and in its discretion, may appoint a “private detective agency” as a special process server in lieu of an individual. It is not necessary that service be made only by a sheriff or PI. Private persons over the age of 18, upon motion, may be appointed by the court to serve original process. 

[Illinois Compiled Statutes §5/2-202]