The Chicago Symphony Orchestra (CSO) is an American symphony orchestra based in Chicago, Illinois. Founded by Theodore Thomas in 1891, the ensemble has been based in the Symphony Center since 1904 and plays a summer season at the Ravinia Festival. Klaus Mäkelä was named music director-designate in 2024, with his first contractual season to begin in 2027. The orchestra's most recent music director is Riccardo Muti, whose tenure spanned the seasons from 2010 to 2023, and he continues to perform on occasion as director-emeritus. The CSO is one of the American orchestras commonly referred to as the "Big Five".