If you qualify for the Illinois Property Tax Credit, you will need your Property Index Number, sometimes called "parcel number" or "permanent index number." This number is located on your county tax bill or assessment notice (for property tax paid on your principal residence during the tax year for which you are filing your return).
If your property tax is paid through your mortgage, you can contact your lender for a copy of your bill. You may also get this number from your county assessor's office.