Chicago ( (listen) shih-KAH-goh, locally also shih-KAW-goh), officially the City of Chicago, is the most populous city in the U.S. state of Illinois, and the third-most populous city in the United States, following New York City and Los Angeles. With a population of 2,746,388 in the 2020 census, it is also the most populous city in the Midwestern United States and the fifth most populous city in North America. Chicago is the county seat of Cook County, the second most populous county in the U.S., while a small portion of the city's O'Hare Airport also extends into DuPage County. Chicago is the principal city of the Chicago metropolitan area, defined as either the U.S. Census Bureau's metropolitan statistical area (9.6 million people) or the combined statistical area (almost 10 million residents), often called Chicagoland. It is one of the 40 largest urban areas in the world.

Located on the shores of freshwater 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; by 1860, Chicago was the youngest U.S. city to exceed a population of 100,000. Even after the Great Chicago Fire of 1871 destroyed several square miles and left more than 100,000 homeless, Chicago's population grew to 503,000 by 1880 — and then doubled to more than a million within the decade. The construction boom accelerated population growth throughout the following decades, and by 1900, less than 30 years after the great fire, Chicago was the fifth-largest city in the world. Chicago made noted contributions to urban planning and zoning standards, including new construction styles (including the Chicago School of architecture), 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 is the site of the creation of the first standardized futures contracts, issued by the Chicago Board of Trade, which today is part of 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 according to tracked data by the Airports Council International. The region also has the largest number of federal highways and is 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. The economy of Chicago is diverse, with no single industry employing more than 14% of the workforce. It is home to several Fortune 500 companies, including Abbott Laboratories, AbbVie, Allstate, Archer Daniels Midland, Boeing, Caterpillar, Conagra Brands, Exelon, JLL, Kraft Heinz, McDonald's, Mondelez International, Motorola Solutions, Sears, United Airlines Holdings, US Foods, and Walgreens.Chicago's 58 million tourist visitors in 2018 set a new record, and Chicago has been voted the best large city in the U.S. for four years in a row by Condé Nast Traveler. The city was ranked first in the 2018 Time Out City Life Index, a global urban quality of life survey of 15,000 people in 32 cities, and was rated second-most beautiful city in the world (after Prague) in 2021. Landmarks in the city include Millennium Park, Navy Pier, the Magnificent Mile, the Art Institute of Chicago, Museum Campus, the Willis (Sears) Tower, Grant Park, the Museum of Science and Industry, and Lincoln Park Zoo. Chicago is also home to the Barack Obama Presidential Center being built in Hyde Park on the city's South Side. Chicago's culture includes the visual arts, literature, film, theatre, comedy (especially improvisational comedy), food, and music, particularly jazz, blues, soul, hip-hop, gospel, and electronic dance music including house music. Of the area's many colleges and universities, the University of Chicago, Northwestern University, and the University of Illinois at Chicago are classified as "highest research" doctoral universities. Chicago has professional sports teams in each of the major professional leagues, including two Major League Baseball teams.