The Illinois Affidavit of Proof of Service of Withholding Notice is a document used by employers to verify that they have provided the necessary notice of income tax withholding to their employees. This document must be completed and signed by the employer and given to the employee, along with a copy of the withholding notice, to certify that the notice was properly served. There are two types of Illinois Affidavit of Proof of Service of Withholding Notice: one for employers located in Cook County, and one for employers located outside of Cook County. Both forms require the employer to provide information about the employee, the date of service, and the manner of service. Additionally, the Cook County form requires that the employer provide additional information regarding the employee’s occupation and Social Security number.