A county sheriff may serve process. A licensed private investigator may also be used (in all counties except Cook), or any citizen over 18 as appointed by the court.

A party may be considered served when he or she receives a copy of the summons personally.

Service may also be effected by leaving a copy at the defendant's home. In this instance, process must be left with a family member or a person residing there who is 13 years or older, and that person must be informed of the contents of the summons. The server must send a copy of the summons in a sealed envelope with postage fully prepaid, addressed to the defendant at his or her home.

After serving process, the process server completes an affidavit of service, which describes the date, place, time and method of service.

If substituted service was used, the server must describe in detail the individual who accepted service for the defendant. The affidavit of service proves that the defendant knows about the lawsuit, and it must be filed with the court before the first appearance of the plaintiff.

Another form of substituted service is service by publication. Service by publication happens when a spouse has disappeared. The petitioner, having made a good faith effort to locate the missing partner, places an advertisement in a newspaper where he or she filed the action. However, this cannot be done without the approval of the court, which requires a diligent search for the missing party.