Chestnut Health Systems is a not-for-profit agency that has cared since 1973 for persons needing behavioral health services. Chestnut provides primary health care, addiction treatment, mental health counseling, housing, and supportive services. It is a leader in researching evidence-based treatment. Chestnut has locations throughout Illinois and in Missouri. It is accredited by The Joint Commission. Chestnut employs more than 750 people.