Northern Illinois is dominated by the metropolitan areas of Chicago, the Quad Cities, and Rockford, which contain a majority (over 75%) of Illinois' population and economic activity, including numerous Fortune 500 companies and a heavy manufacturing, commercial, retail, service, and office based economy.

Additional smaller cities in this area include Kankakee, LaSalle-Peru, Ottawa, Freeport, Dixon, and Sterling-Rock Falls, which still have predominantly manufacturing and agricultural economies.

Northern Illinois is also one of the world's busiest freight railroad and truck traffic corridors.

Interstate 88 (the Ronald Reagan Memorial Tollway) connects the region, east–west, stretching from the Quad Cities, eastward through Sterling-Rock Falls, Dixon, DeKalb, Aurora, Naperville, and into Chicago.

Such significant developments in science including the creation of the Atomic Bomb and the Fujita Scale were rooted in Northern Illinois institutions.

Northern Illinois also has large fanbases for the Illinois Fighting Illini, Notre Dame Fighting Irish, Iowa Hawkeyes, and the Northwestern Wildcats, as well Chicago's professional sports teams such as the Sox, the Cubs, the Bears, the Blackhawks, and the Bulls.

Those west of McHenry and Kane counties have more stereotypical Midwestern dialects, and might not be able to be distinguished from people in Iowa or Nebraska.

Its metropolitan area, sometimes called Chicagoland, is home to 9.5 million people and is the third-largest in the United States.

The collar counties (DuPage, Kane, Lake, McHenry, and Will) are tied to Chicago economically, but, like many suburban areas in the United States, have very different political leanings than does the core city.

[6] Because Chicago is so firmly entrenched in the Democratic column, and rural Downstate Illinois is so overwhelmingly Republican, the collar counties are routinely cited as being the key to any statewide election.

[7][8][9] However, that conventional wisdom was challenged by the fact that in 2010 Democrat Pat Quinn became governor while winning only Cook, St. Clair, Jackson and Alexander counties.

These cities, Davenport and Bettendorf (in Iowa) and Rock Island, Moline, and East Moline (in Illinois), are the center of the Quad Cities Metropolitan Area, which, as of 2012, had a population estimate of 382,630 and a CSA population of 474,226, making it the 90th largest CSA in the nation.

[18][19] The Quad Cities is midway between Minneapolis and St. Louis, north and south, and Chicago and Des Moines, east and west.