The Illinois Certificate of Service is a legal document used in the state of Illinois to prove that a court filing has been served on the opposing party. It is also used to document the fact that the opposing party has been notified of the filing. It is signed by the party who initiated the action, or by the individual who served the documents. There are two types of Illinois Certificate of Service: the Affidavit of Service and the Certificate of Service. The Affidavit of Service is signed by the person who served the documents and attests that the documents were delivered to the opposing party. The Certificate of Service is a more detailed document and is signed by the party who initiated the action. It states that the documents were served in accordance with the applicable rules of court.