Chicago is the largest city in the state of Illinois, the largest in the Midwest, and, with a population of nearly 3 million people located almost entirely in Cook County (a portion of the city's O'Hare International Airport overlaps into DuPage County), is the third-most populous city in the United States. The Chicago metropolitan area (commonly referred to as Chicagol and) has a population of over 9.7 million people in Illinois, Wisconsin and Indiana, making it also the third largest metropolitan area in the U.S. Adjacent to Lake Michigan, it is the largest city located on the Great Lakes and among the world's twenty-five largest urban areas by population. Chicago has been classified as an alpha world city for its worldwide economic influence.

Incorporated as a city in 1837 after being founded in 1833 at the site of a portage between the Great Lakes and the Mississippi River watershed, it soon became a major transportation hub in North America and quickly became the business and financial capital of the American Midwest. Since the Chicago World's Fair of 1893, it has been regarded as one of the ten most influential cities in the world. Among the fields in which its influence has been seen are physics where Chicago Pile-1 served as the world's first artificial nuclear reactor, economics and architecture where it has contributed the Chicago school of architecture. Home of the earliest skyscrapers, it today boasts some of the world's tallest buildings, including the Sears Tower, Aon Center, and John Hancock Center, plus the under-construction Chicago Spire and Trump International Hotel and Tower. The University of Chicago is a leader in many fields and has contributed its own Chicago schools such as Chicago school economics.