If your spouse doesn't live in Illinois and you file for divorce here (presumably, you meet the residency requirements to file here), you still need to have them served. The clerk of the court in the county where you file (Cook County, for instance) will issue what is known as a Summons. Arrangements are then made with the sheriff of the county in which your spouse lives, to have the Illinois issued summons, served over there. If you don't know where your spouse is living, you may be able to have them served by publication - or through a newspaper. Doing it that way, however, can get you divorced but might impact your ability under Illinois law to have property issues, etc addressed by the court. Depending on the circumstances of your marriage, that could be an issue.
Divorce laws can get tricky, not just when it comes to service but for procedural matters in general. If you are wondering how to divorce your out of state spouse, give us a call at 630 250-8813 to schedule a confidential office appointment to discuss your particular circumstances.