The Chicago Tribune is a major daily newspaper based in Chicago, Illinois, United States, and the flagship publication of the Tribune Company. Formerly self-styled as the "World's Greatest Newspaper" (for which WGN radio and television is named), it remains the most read daily newspaper of the Chicago metropolitan area and the Great Lakes region and is currently the eighth largest newspaper in America by circulation (and the second largest under Tribune's ownership behind Los Angeles Times). Traditionally published as a broadsheet, on January 13, 2009, the Tribune announced it would continue publishing as a broadsheet for home delivery, but would publish as a tabloid for newsstand, news box, and commuter station sales.

On April 2, 2007, the Tribune Company announced a buy-out plan led by Chicago real estate magnate Sam Zell worth $8.2 billion, associated with a stock buyback at $34 per share, and an Employee Stock Ownership Plan. The deal closed on December 20, 2007, with Zell as the company's new chairman. As part of the deal, the Chicago Cubs and their park, Wrigley Field, will be sold, as well as the Tribune Company's share of Comcast SportsNet Chicago. Less than a year after the deal closed, the Tribune Company filed for Chapter 11 bankruptcy protection on December 8, 2008.

The Tribune was founded by James Kelly, John E. Wheeler and Joseph K.C. Forrest, publishing its first edition on June 10, 1847. The paper saw numerous changes in ownership and editorship over the next eight years. Initially, the Tribune was not politically affiliated but tended to support either the Whig or Free Soil parties against the Democrats in elections. By late 1853 it was frequently running xenophobic editorials that criticized foreigners and Roman Catholics. About this time it also became a strong proponent of temperance. However nativist its editorials may have been, it was not until February 10, 1855 that the Tribune formally affiliated itself with the nativist American or Know Nothing party, whose candidate Levi Boone was elected Chicago mayor the following month.