Illinois FAQ - Who is required to file an Illinois state tax return?

If you're an Illinois resident, must file Form IL-1040, Individual Income Tax Return, if:

You filed a federal income tax return;

You were not required to file a federal income tax return, but your Illinois base income, Line 11, is greater than your Illinois exemption allowance.

An Illinois resident taxpayer who worked in Iowa, Kentucky, Michigan, or Wisconsin must file Form IL-1040 and include as Illinois income any compensation you received from an employer in these states. Compensation paid to Illinois residents working in these states is taxed by Illinois. Based on reciprocal agreements between Illinois and these states, these states do not tax the compensation of Illinois residents.

If your employer in any of these states withheld that state’s tax from your compensation, you may claim a refund of that state’s tax withheld by filing the correct form with that state. You may not claim tax withheld by an employer in these states as a credit on your Illinois return.

If you are a part-year resident (i.e., you were a resident of Illinois for part, but not all of the year), you must file Form IL-1040, Individual Income Tax Return and Schedule NR, Nonresident, and Part-Year Resident Computation of tax, if:

Your earned income from any source while you were a resident

Your earned income from Illinois sources while you were not a resident, or

You want a refund of any Illinois Income Tax was withheld.

If you were a nonresident (you were not an Illinois resident at any time during the tax year), you must file Form IL-1040, Individual Income Tax Return and Schedule NR, Nonresident, and Part-year Resident Computation of tax, if:

you earned enough taxable income from Illinois sources to have a tax liability, or

you want a refund of any Illinois Income Tax was withheld in error. You must attach a letter of explanation from your employer.

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