A trust is different than an individual for state tax filing.  If the trust is domiciled in Illinois, or the trust activity is in Illinois, or possibly even the trustee lives in Illinois the trust would file a state trust tax return there.  State taxes for trusts can be confusing especially when the trustor moves later in life to be near their kids, or the trustee moves for whatever reason. You have to start with the trust document and then possibly look at [each] state rules.  Not always a clear-cut answer.