iO, or iO Chicago, (formerly known as "ImprovOlympic") is a theater and training center located at 1501 N. Kingsbury in Chicago, Illinois (formerly 3541 N. Clark St.). The theater both teaches and hosts performances of improvisational comedy. It was founded in 1981 by the late Del Close and Charna Halpern. iO concentrates on "long-form" improvisational structures, in contrast to the "short-form" or "improv game" format of Theatresports, ComedySportz or the television show Whose Line Is It Anyway?. The iO's signature piece is the "Harold", and the theater features other forms of improvisation, as well as sketch comedy and stand-up comedy.