|This article relies on references to primary sources. (March 2009)|
iO, or iO Chicago, (formerly known as "ImprovOlympic") is a theater located at 3541 N. Clark St., in Chicago, Illinois, one-half block south of Wrigley Field, home of the Chicago Cubs baseball team. The theater both teaches and hosts performances of improvisational comedy. It was founded in the 1980s by Del Close and Charna Halpern. iO concentrates on "long-form" improvisational structures, in contrast to the "short-form" or "improv game" format of Theatresports or the television show Whose Line Is It Anyway?. The iO's signature piece is the "Harold".
The theater consists of two performing spaces: the downstairs "Cabaret", which is a cabaret-style minimal stage, and the upstairs "Del Close Theater", which is a more traditional theater-style performance space with a backstage area and entrances/exits on the set. There are full bars located in both spaces.
There are performances both upstairs and downstairs every night of the week, with the exception of some holidays. Shows can consist of either a Harold or a "show". Shows can be special limited runs (typically 1- 2 months) of a particular group / piece, or they can be open runs of long standing formats using regular performers at the theater (e.g. The Armando Diaz Theatrical Experience and Hootenanny, which has the same format but a different cast every week). A typical "Harold performance" consists of 2 or 3 groups doing a Harold; newer teams usually open the night and more veteran teams give the final performance(s). On some nights, usually in the downstairs theater, they will play the short form game "The Dream" using an audience volunteer after the second Harold of the night, and then play the short form game "Freeze" after the final Harold. A "show" slot typically, but not always, has a Harold team open for them, usually one of the newer teams.
There are approximately 25 Harold teams at iO at any given time; the exact number fluctuates up and down when old teams are cut from the roster and new teams are created. New teams in the past have come from the final performance level class (5B), from open auditions, and from recombining performers taken from cut teams. The oldest Harold team has been together for more than 5 years, several have been around for 3+ years, and there are many that are less than 6 months old.
The Reckoning is one of the theater's most well known Harold Teams and has the distinction of being the only team to maintain all of its original members. The group no longer performs a traditional Harold and instead creates and explores original and experimental forms.
1981 - The ImprovOlympic was created, putting competing teams of comedic improvisers on stage in front of audiences. It was the brainchild of David Shepherd, who used early Viola Spolin theater games 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.
1983 - Shows began shifting to a long-form approach.
1995 - The ImprovOlympic moves to its current location on Clark St. in Chicago.
2001 - The International Olympic Committee threatened the theater legally over its use of the name "ImprovOlympic" and the name was subsequently changed to "iO."
2005 (Sept 2) - iO holds its 25th anniversary show at the Chicago Theater in downtown Chicago. The wireless microphones go dead shortly into the show, but the improvisers rally and play using wired mics for the rest of the performance. Celebrity veterans of the iO program who return to play include Mike Myers, Tim Meadows, Amy Poehler, Ike Barinholtz, and many more. The opening to the Harold piece performed is conducted by the most veteran iO house team "The Reckoning."
2006 - iO begins a joint venture with ComedyWorx of Raleigh, NC to create the third iO training center, named iO South.
Current Shows 
Notable alumni