The Employee's Illinois Withholding Allowance Certificate, Illinois Form IL-W4, must be completed so that you know how much state income tax to withhold from your new employee's wages. The importance of having each employee file a state withholding certificate as well as a federal Form W-4 cannot be overstated, so make its completion a priority.
If for some reason an employee does not file one, you must withhold tax as if the employee had claimed no exemptions. If a married employee does not claim his married status on the form, the employer must withhold on the basis of the "single employee" withholding tables. Since the withholding rates for single employees are higher than those for married workers and since the amount of tax withheld from a given wage payment decreases as the number of exemptions claimed increases, it is important that you ask all employees to file the proper certificates.
The file is in Adobe Portable Document Format (.pdf), and you will need the free Acrobat Reader to view and print the file.
State Income Tax Withholding Map