The Illinois Confederation,  sometimes referred to as the Illiniwek or Illini, was a group of 12–13 Native American tribes in the upper Mississippi River valley of North America. The tribes were the Kaskaskia, Cahokia, Peoria, Tamaroa, Moingwena, Michigamea, Chepoussa, Chinkoa, Coiracoentanon, Espeminkia, Maroa, and Tapouara.  At the time of European contact in the 17th century, they were believed to number over 10,000 people.  Most of the Illinois spoke various dialects of the Miami-Illinois language, one of the Algonquian languages family, with the known exception of the Siouan-speaking Michigamea. They occupied a broad inverted triangle from modern-day Iowa to near the shores of Lake Michigan in modern Chicago south to modern Arkansas. By the mid-18th century, only five principal tribes remained: the Cahokia, Kaskaskia, Michigamea, Peoria, and Tamaroa.
The Illinois autonym was not "Illinois," but rather "Inoka," a word of unknown meaning. The name "Illinois" ultimately derives from the Miami-Illinois term irenweewa "s/he speaks normally" or "s/he speaks in the ordinary way." The term was likely originally applied to the Illinois by the Miami tribe, who spoke a dialect of the same language. It was then borrowed by Odawa as ilin(i)we/alin(i)we, and loaned from Odawa into French, and from there into English (in the French of the 1600s, the spelling <Illinois> represented /ilinwe/). 
When French explorers first journeyed to the region from Canada in the 17th century, they found the area inhabited by a vigorous, populous, Algonquian-speaking nation. What we know today about the Illinois is based on the historical account Jesuit Relations, written by French Jesuits. The missionaries who lived among the various native nations wrote the Relations and sent the reports back to their superiors in France. One name for an Illinois Confederation tribe, the Cahokia, was used as a name for a French settlement, now Cahokia, Illinois, near what are now called the Cahokia Mounds, the remains of a large pre-Columbian city. However, it is currently unknown whether the Illinois Confederation peoples, including the Cahokia, have any relationship to the earlier native builders of the mounds civilization.
In the 17th century, the Illinois suffered from a combination of exposure to Eurasian infectious diseases, to which they had no natural immunity, and warfare by the expansion of the Iroquois into the western Great Lakes region. The Iroquois had hunted out their traditional lands and sought more productive hunting and trapping areas (see, Beaver Wars). They sought furs to purchase European goods in the fur trade. Many of the Illinois migrated to present-day eastern Kansas to escape the pressure from other tribes and encroaching European settlers.
Benjamin Drake, writing in 1848, records that the Michigamie, along with the other bands in the Illinois Confederation, had been attacked by a general confederation of the Sauk, Fox, Sioux, Chippewa, Ottawa, and Potawatamies, along with the Cherokee and Choctawa from the south. The war continued for a great many years until the Illinois Confederation was destroyed. Drake records that by 1826 only about 500 members of the Confederation remained. Lieutenant Pike, in his travels to the sources of the Mississippi, remarked, "By killing the celebrated Sauk chief, Pontiac, the Illinois, Cahokias, Kaskaskias, and Peorias kindled a war with the allied nations of the Sauks and Reynards, which has been the cause of the almost entire destruction of the former nations." 
The Illinois lived in a seasonal cycle related to cultivation of domestic plants and hunting, with movement from semi-permanent villages to hunting camps. They seasonally lived in long houses and wigwams of wood and woven mats.  They planted crops of maize (corn), beans, and squash, known as the " Three Sisters". They prepared dishes such as sagamite. They also gathered wild foods such as nuts, fruit, roots, and tubers. In the hunting season, the men hunted bison, deer, elk, bear, cougar, lynx, turkey, geese and duck. Women prepared the meat for preservation and the hides for equipment and clothing. They tapped maple trees and made the sap into a drink or boiled it for syrup and sugar. 
As a consequence of the Indian Removal Act, in the 1830s, the Illinois were relocated from where they had migrated to in eastern Kansas to northeastern Indian Territory. Today they chiefly reside in Ottawa County, Oklahoma, as the Peoria Tribe of Indians of Oklahoma.
- The Indian Tribes of North America, by John R. Swanton. Bulletin (Smithsonian Institution; Bureau of American Ethnology), 145.
- The Illinois: Identity, Illinois State Museum, 2000
- "Native Americans:Historic:The Illinois:History:The Illinois Decline". Museum.state.il.us. Retrieved 2013-09-14.
- Costa, David J. 2008. "On the Origins of the Name "Illinois"", Le Journal 24/4: 6-10.
- Drake, Benjamin (1848). The Life and Adventures of Black Hawk: with Sketches of Keokuk, the Sac and Fox Indians, and the Late Black Hawk War. H.S. & J. Applegate & Co. p. 27.
- "Native Americans:Historic:The Illinois:Technology:Houses". www.museum.state.il.us. Retrieved 14 April 2018.
- "The Illiniwek", The Lewis and Clark Journey of Discovery, National Park Service, accessed 29 Sep 2009
- Costa, David J. 2000. "Miami-Illinois Tribe Names". In John Nichols, ed., Papers of the Thirty-first Algonquian Conference 30-53. Winnipeg: University of Manitoba.
- Costa, David J. 2008. "On the Origins of the Name "Illinois"." Le Journal 24/4: 6-10.
- Costa, David J.; Wolfart, H.C., ed. (2005). "The St-Jérôme Dictionary of Miami-Illinois" (PDF). Papers of the 36th Algonquian Conference. Winnipeg: University of Manitoba. pp. 107–133. Archived from the original (PDF) on July 27, 2011. Retrieved March 7, 2012.
- Masthay, Carl, editor. Kaskasia Illinois-to-French Dictionary. St. Louis, Missouri: Carl Masthay. p. 757. ISBN 0-9719113-04.
- NPS Site on the Illiniwek
- Some Account of the Indian Tribes Formerly Inhabiting Indiana and Illinois
- The Illinois - State Museum of Illinois
- Tribes of the Illinois/Missouri Region at First
- The Tribes of The Illinois Confederacy
- Peoria Tribe of Indians of Oklahoma
- Lenville J. Stelle, Inoca Ethnohistory Project: Eye Witness Descriptions of the Contact Generation, 1667 - 1700
- Encyclopedia of Oklahoma History and Culture - Illinois
Texts on Wikisource:
- " Illinois, a confederacy of five tribes of North American Indians". Collier's New Encyclopedia. 1921.
- " Illinois Indians". The New Student's Reference Work. 1914.
- Mooney, James (1913). " Illinois Indians". Catholic Encyclopedia.
- Shea, J. G. (1879). " Illinois, a tribe of North American Indians". The American Cyclopædia.