Note that this list refers only to the population of individual municipalities within their defined limits, which does not include other municipalities or unincorporated suburban areas within urban agglomerations. A different ranking is evident when considering metropolitan area populations.
| Rank | City | County | Population (2008 est.)[1] | Population (2000 Census)[1] | Change (%) |
| 1 | Chicago | Cook, DuPage | &0000000002853114.0000002,853,114 | &0000000002896016.0000002,896,016 | -1.5% |
| 2 | Aurora | Kane, DuPage, Kendall, Will | &0000000000171782.000000171,782 | &0000000000142990.000000142,990 | 20.1% |
| 3 | Rockford | Winnebago | &0000000000157272.000000157,272 | &0000000000150115.000000150,115 | 4.8% |
| 4 | Joliet | Will, Kendall | &0000000000146125.000000146,125 | &0000000000106221.000000106,221 | 37.6% |
| 5 | Naperville | DuPage, Will | &0000000000143117.000000143,117 | &0000000000128358.000000128,358 | 11.5% |
| 6 | Springfield | Sangamon | &0000000000117352.000000117,352 | &0000000000111454.000000111,454 | 5.3% |
| 7 | Peoria | Peoria | &0000000000114114.000000114,114 | &0000000000112936.000000112,936 | 1% |
| 8 | Elgin | Kane | &0000000000106330.000000106,330 | &0000000000094487.00000094,487 | 12.5% |