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.
The building has four performance spaces:
The Del Close Theater - This is the second-largest of iO's theaters, and is located on the ground floor of the building. It hosts some of iO's premiere shows, such as Virgin Daiquiri, The Improvised Shakespeare Company, and The Deltones.
The Chris Farley Cabaret - One of two cabaret-style theaters in the building, this hosts many different types of performances, mostly improvised and mostly non-Harold.
The Jason Chin Harold Cabaret - Named after a revered teacher at iO who died on January 8, 2015, this theater predominantly features performances of the Harold from house teams at the theater which specialize in that form.
The Mission Theater - The largest theater at iO, formerly run and managed by T. J. Jagodowski and David Pasquesi, or "TJ and Dave", now features some of iO's most iconic and longest-running shows such as The Armando Diaz Theatrical Experience and Hootenanny, The Musical Armando, and Whirled News Tonight.
The ImprovOlympic was created in 1981 putting competing teams of comedic improvisers on stage in front of audiences. This was the brainchild of David Shepherd who originally created the format in 1972 in New York with Howard Jerome. David Shepherd used the Theater Games, created earlier by Viola Spolin, as a way for teams to compete. The first ImprovOlympic classes and shows took place at The Players Workshop in Chicago, where Charna Halpern was an improv student. Charna Halpern became David Shepherd's assistant, and eventually the producer of the competitions. There were also competitions at a network of local bars and clubs. In 1982, The ImprovOlympic moved from The Players Workshop to its own space. Teams began to form out of every major improv troupe in Chicago. Shows began shifting to a long-form approach by 1983.
In 1995, The ImprovOlympic moved to its location on Clark St. in Chicago. An additional theater, iO West was opened by Paul Vaillancourt in Los Angeles, California in 1997. Today it is managed by Colleen Doyle and Zach Huddleston. In 2001, The International Olympic Committee legally threatened the theater over its use of the name "ImprovOlympic" and the name was subsequently changed to "iO." On September 2, 2005, iO held its 25th anniversary show at the Chicago Theatre in downtown Chicago. The wireless microphones went dead shortly into the show, but the improvisers rallied and played using wired mics for the rest of the performance. Celebrity veterans of the iO program who returned to play included Mike Myers, Tim Meadows, Amy Poehler, and Ike Barinholtz. The opening to the Harold piece performed was conducted by the most veteran iO house team "The Reckoning."
In August 2014, after almost 20 years in Chicago's Wrigleyville neighborhood, Charna Halpern bought a building in the Lincoln Park neighborhood and moved the iO Theater to its new home at 1501 N. Kingsbury St.