At least 10,000 years ago, prehistoric people came to what is now Illinois. Archeological excavations at the Koster Site in the lower Illinois River valley have revealed evidence of almost continuous habitation there by advanced Indian cultures from about 8000 B.C. to 1200 A.D. The height of Indian civilization in Illinois was the Middle Mississippian culture, which flourished from 900 A.D. to about the late 15th century. The Indians of this period were mound builders who lived along the Illinois, Mississippi, Ohio, and Wabash rivers. When Europeans first came in the 17th century, the area was inhabited by primitive, seminomadic Indians called the Illinois or Illiniwek.

Important dates in Illinois
1673 Louis Jolliet of Canada and Jacques Marquette of France were probably the first Europeans in Illinois.
1699 French priests founded a settlement in Cahokia, the oldest town in Illinois.
1717 Illinois became part of the French colony of Louisiana.
1763 France included Illinois in the territory it ceded to Britain after the French and Indian War.
1778 George Rogers Clark's forces captured Cahokia and Kaskaskia during the Revolutionary War. The Illinois region became a county of Virginia.
1783 The Illinois region became part of the United States under the treaty ending the Revolutionary War.
1784 Virginia gave up its claim to Illinois to the national government.
1787 Congress made Illinois part of the Northwest Territory.
1800 Illinois became part of the Indiana Territory.
1809 Congress made Illinois a territory.
1818 Illinois became the 21st state on December 3.
1818 Nathaniel Pope, the Illinois territorial delegate to the U.S. Congress, succeeded in having the state’s northern border extended to its present position.
1848 The completion of the Illinois and Michigan Canal provided a water connection between the Great Lakes and the Mississippi Valley.
1858 Abraham Lincoln and Stephen A. Douglas debated throughout Illinois in their senatorial campaigns.
1871 The Chicago Fire destroyed much of the city.
1886 Discontent among laborers led to the Haymarket Riot in Chicago.
1893 The World’s Columbian Exposition, an elaborate fair held in Chicago, brought attention to the city’s accomplishments.
1900 The Chicago Sanitary and Ship Canal was completed, making the Chicago River flow backward.
1920's Illinois built a network of hard-surfaced roads.
1933-1934 The Century of Progress Exposition was held in Chicago.
1942 Scientists at the University of Chicago controlled an atomic chain reaction for the first time.
1960 One of the country's largest nuclear reactors was completed at Morris.
1965 A panel of federal and state judges reapportioned the state Senate. A special commission reapportioned the state House of Representatives.
1969 Illinois adopted individual and corporate income taxes.
1970 Illinois voters approved a new constitution, which went into effect July 1, 1971.
1986 James R. Thompson became the first Illinois governor to be elected to a fourth term.
1993 Floods caused heavy damage in Illinois.
2005 Barack Obama of Illinois became the only African American in the U.S. Senate.