If you are an employee, you must complete form IL-W-4, Employee's Illinois Withholding Allowance Certificate and Instructions, so your employer can withhold the correct amount if Illinois Income Tax from your pay. The amount withheld from your pay depends in part on the number of allowances you claim on this form.
If you do not file a completed Form IL-W-4 with your employer, fail to sign the form or include necessary information, or alter the form, your employer will withhold Illinois Income Tax on the entire amount of your compensation, without allowing any exemptions.