The area of Illinois was part of the original territory of the United States, being part of lands ceded by four states to the United States and designated in 1787 as the "Territory northwest of the River Ohio." Illinois Territory was organized on February 3, 1809, from the western part of Indiana Territory, and included all of present-day Illinois, most of Wisconsin, the western part of the Upper Peninsula of Michigan, and northeastern Minnesota.

Although the territory had not yet been legally established, census data for Illinois are available beginning with the 1800 census. The 1810 population is for the entire area of the Illinois Territory, including territory not in the present state. For an explanation of the revision to the 1800 population of Illinois, see Richard L. Forstall, Population of States and Counties of the United States: 1790-1990, Washington, DC: U.S. Government Printing Office, 1996, page 48.

Data for the legally established state of Illinois are available beginning with the 1820 census.

Metropolitan And Micropolitan Statistical Areas And Related Statistical Areas

Illinois has 12 metropolitan statistical areas, 22 micropolitan statistical areas, 2 metropolitan divisions, and 7 combined statistical areas.

Counties

There are 102 counties in Illinois.  All counties are functioning governmental units; each is governed by a board with between 5 and 29 members.

County Subdivisions

There are 1,710 county subdivisions in Illinois known as minor civil divisions (MCDs); 85 counties have functioning, but not necessarily active, townships and 17 counties have nonfunctioning election precincts.  There are 1,432 townships and 274 election precincts.  Chicago city is independent of any MCD and serves as an MCD equivalent in two counties, creating two county subdivisions.  There are also two undefined MCDs that cover the Illinois portion of Lake Michigan.  Alexander, Calhoun, Edwards, Hardin, Johnson, Massac, Menard, Monroe, Morgan, Perry, Pope, Pulaski, Randolph, Scott, Union, Wabash, and Williamson counties have election precincts.  All other counties have townships.  The governing body for each township is an elected township supervisor and a board of trustees.

Places

Illinois has 1,367 places, 1,299 incorporated places and 68 census designated places (CDPs). The incorporated places consist of 300 cities, 16 towns, and 983 villages. The minimum population to incorporate as a city is 2,500.  For a village the minimum population for incorporation ranges from 200 (in counties with a population less than 150,000) to 2,500.  Incorporated places are dependent within county subdivision with the following exceptions:

• Independent of any township
  • Chicago city
• Coextensive with a single township
  • Alton
  • Belleville
  • Berwyn
  • Bloomington
  • Champaign
  • Cicero
  • East St. Louis
  • Evanston
  • Freeport
  • Galesburg
  • Godfrey
  • Macomb
  • Oak Park
  • Quincy
  • River Forest
  • Urbana
  • Warsaw
  • Zion
• Coextensive with a single precinct
  • Cairo
  • Golconda
  • Valmeyer
• Coextensive with multiple whole precincts
  • Jacksonville
  • South Jacksonville
  • Waterloo
• Coextensive with one more precincts in Monroe County, but also has part of one township in St. Clair County
  • Columbia city
  • Hecker village
• Contain one or more whole precincts and parts of others
  • Du Quion
  • Metropolis
  • Petersburg
  • Pinckneyville

Census Tracts/Block Groups/Blocks

Illinois has 3,123 census tracts, 9,691 block groups, and 451,554 census blocks.

Congressional Districts

For the 111^th Congress (January 2009-January 2011), Illinois had 19 congressional districts.  For the 113^th Congress (January 2013-January 2015), Illinois has 18 congressional districts as a result of reapportionment based on the 2010 Census.

School Districts

There are 379 elementary school districts, 104 secondary school districts, and 389 unified school districts in Illinois. 

State Legislative Districts

There are 59 state senate districts and 118 state house districts in Illinois. There is also one state house district undefined and one state senate district undefined each comprising the portion of Illinois in Lake Michigan.

Urban Areas

Illinois has 158 urban areas; 19 urbanized areas and 139 urban clusters.

Zip Code Tabulation Areas

There are 1,384 ZIP Code tabulation areas (ZCTAs) in Illinois.

¹  Source:  U.S. Department of Commerce, Bureau of the Census, Fourteenth Census of the United States, 1923
²  Source:  U.S. Census Bureau, recomputation for historical county level data which relied upon aggregate county level population data with an estimated county centroid resulting in a possible error of up to one mile.
³  Source:  U.S. Department of Commerce, Bureau of the Census, Centers of Population for States and Counties, 1974
⁴  Source:  U.S. Census Bureau, Geography Division, recomputation from archived national block group/enumeration area data resulting in a possible error of up to 1,000 feet.
⁵  Source:  U.S. Census Bureau, Geography Division, recomputation from archived national block group data resulting in a possible error of up to 1,000 feet.
⁶  Source:  U.S. Census Bureau, computation from national block-level data