Chicago is the most populous city in the U.S. state of Illinois and the third-most populous in the United States. With a population of 2,746,388 in the 2020 census, it is also the most populous city in the Midwest. As the seat of Cook County, the second-most populous county in the U.S., Chicago is the centre of the Chicago metropolitan area. On the shore of Lake Michigan, Chicago was incorporated as a city in 1837 near a portage between the Great Lakes and the Mississippi River watershed. It grew rapidly in the mid-19th century. The Great Chicago Fire in 1871 destroyed several square miles and left more than 100,000 homeless, but Chicago's population continued to grow. Chicago made noted contributions to urban planning and architecture, such as the Chicago School, the development of the City Beautiful Movement, and the steel-framed skyscraper.

Chicago is an international hub for finance, culture, commerce, industry, education, technology, telecommunications, and transportation. It has the largest and most diverse derivatives market in the world, generating 20% of all volume in commodities and financial futures alone. O'Hare International Airport is routinely ranked among the world's top six busiest airports by passenger traffic; the region is also the nation's railroad hub. The Chicago area has one of the highest gross domestic products (GDP) in the world, generating $689 billion in 2018. Chicago's economy is diverse, with no single industry employing more than 14% of the workforce.

Chicago is a major tourist destination. Chicago's culture has contributed much to the visual arts, literature, film, theater, comedy (especially improvisational comedy), food, dance, and music (particularly jazz, blues, soul, hip-hop, gospel, and electronic dance music, including house music). Chicago is home to the Chicago Symphony Orchestra and the Lyric Opera of Chicago. The Chicago area also hosts the University of Chicago, Northwestern University, and the University of Illinois Chicago, among other institutions of learning. Chicago has professional sports teams in each of the major professional leagues, including two Major League Baseball teams.