As of January 2016, Grounds for divorce in Illinois no longer exist. There were formerly various requirements as to the proof necessary to establish grounds, but the only reason, and necessary allegation for a divorce, is that irreconcilable differences have caused the irretrievable breakdown of the marriage. The issues that may have caused, or lent themselves to the reason for the divorce, can still be helpful to prove other contested issues. For example, adultery is not grounds for divorce, but if a parent spent time away from the child to spend that time in an extra-marital affair, that choice to spend time away from the child could be useful in arguing who should be the primary decision maker.
The court only has to find that “Irreconcilable differences have caused the irretrievable
breakdown of the marriage and the court determines that efforts at reconciliation have failed or that future attempts at reconciliation would be impracticable and not in the best interests of the family.” 750 ILCS 5/401(a)