| 1891 | Basketball invented at Springfield, Mass., by James Naismith. |
| 1893 | Chicago (Morgan Park Academy) becomes first Illinois secondary school to organize a boys basketball team. |
| 1900 | Chicago (Englewood) defeats Elgin (H.S.) in the first known boys basketball game between Illinois high schools. |
| 1904 | Team from Chicago (North Division) plays in exhibition tournament of high school basketball teams at St. Louis Olympics. |
| 1908 | Lewis Omer, athletic director of Oak Park (O.P.-River Forest), gains IHSAA approval to organize first state high school basketball tournament, an invitational meet held at Oak Park YMCA; Peoria (H.S.) defeats Rock Island (H.S.) in title game. |
| 1909 | IHSAA assumes administration of tournament and opens it to all member schools; state final tournament moved to Bloomington YMCA; six district tournament champions and six runners-up advance to state final. |
| 1910 | State final field reduced to six teams as district champions only allowed to advance. |
| 1911 | State final tournament moved to Bradley Gym in Peoria. |
| 1912 | State final tournament moved to Decatur YMCA. |
| 1913 | State final tournament returned to Bradley Gym. |
| 1914 | State final tournament returned to Decatur YMCA; state final field expanded to eight teams. |
| 1915 | State final tournament moved to Millikin Gym in Decatur. |
| 1916 | State final field expanded to 11 teams. |
| 1917 | State final field expanded to 12 teams. |
| 1918 | State final tournament moved to Springfield H.S.; state final field expanded to 13 teams. |
| 1919 | After eleven years at various sites, state final tournament moved to campus of University of Illinois at Urbana-Champaign; games played at Men's Gym Annex (now known as Kenney Gym). |
| 1920 | State final field expanded to 16 teams. |
| 1921 | Cumbersome field of 21 district tournament winners qualifies for state finals; at end of school year, Charles Whitten hired as IHSAA's first executive secretary and given duty of reorganizing tournament. |
| 1922 | Sectional tournament stage added between district and state final; state final field reduced to four teams. |
| 1925 | Elgin (H.S.) becomes first school to win back-to-back state titles. |
| 1926 | State final tournament moved to Huff Gym. |
| 1927 | State final field expanded to eight teams; Cicero (Morton) wins national championship tournament at University of Chicago (after being eliminated in sectional during state tournament). |
| 1928 | Chicago Public League teams enter state tournament for first time. |
| 1933 | State final field expanded to sixteen teams, known as "Sweet Sixteen." |
| 1936 | Regional tournament stage added between district and sectional; lowest-rated teams in each area required to play in district tournament; district and regional runners-up allowed to advance to next stage. |
| 1937 | Chicago Public League champion given automatic berth in the state final tournament. |
| 1938 | First tournament played under new rule eliminating center jump after each field goal; district tournament definition changed so that smallest schools in each area, rather than the lowest-rated schools, required to play in district tournament. |
| 1939 | IHSAA assistant executive secretary H.V. Porter coins the term "March Madness" to describe state basketball tournament. |
| 1940 | IHSAA (Illinois High School Athletic Association) becomes IHSA (Illinois High School Association). |
| 1941 | In one-year experiment, state final field reduced to four teams; first-round and quarterfinal games held at four sites; private schools allowed to join IHSA and enter tournaments; Chicago Public League refuses to send champion to state final in dispute over new IHSA constitution. |
| 1942 | Segregated schools of southern Illinois, never allowed to participate in the state tournament, invited to stage conference championship game prior to the first quarterfinal game at Huff Gym. |
| 1943 | District and regional runners-up no longer allowed to advance to next stage. |
| 1944 | Taylorville becomes first known undefeated state champion. |
| 1946 | Segregated schools of southern Illinois allowed to compete in state tournament; special conference championship game discontinued. |
| 1949 | State final carnival held for first time. |
| 1952 | Hebron becomes only district (small) school to win state championship; first live television coverage of state final (semifinals and title game). |
| 1956 | State final field reduced to eight teams, known as "Elite Eight"; first-round games of state final played at eight super-sectional sites. |
| 1958 | Chicago (Marshall) becomes first Chicago school to win state basketball title. |
| 1963 | State final tournament moved to Assembly Hall. |
| 1972 | Tournament split into Class A (small schools), won by Lawrenceville, and AA (large schools), won by Dolton (Thornridge); district tournament stage eliminated; two state finals held on consecutive weekends at Assembly Hall; state final carnival discontinued. |
| 1975 | Chicago Catholic League teams enter state tournament for first time. |
| 1988 | First tournament played under new rule allowing three-point field goal. |
| 1989 | In only triple-overtime championship game, East St. Louis (Lincoln) becomes first school to win three consecutive state titles. |
| 1991 | Class AA teams seeded in sectional complexes instead of at the regional stage; regional games played on court of higher seed. |
| 1992 | "Happening" three-point and slam-dunk contests held for first time; state final reunions held for first time. |
| 1996 | After 77 years at the University of Illinois, state final tournament moved to Peoria Civic Center; March Madness Experience held for first time; reunions discontinued. |
| 1997 | Peoria (Manual) becomes first school to win four consecutive state championships. |
| 2003 | Chicago Public League champion no longer given automatic berth in Class AA state finals; Public League teams enter tournament at regional level in either Class A or Class AA; Class AA teams seeded in modified sectional complex with fixed regional sites. |
| 2004 | Class A teams seeded in modified sectional complex with fixed regional sites, divided into two sub-sectionals. |
| 2005 | Some Class AA sectionals seeded in two sub-sectionals. |
| 2020 | Tournaments cancelled due to COVID-19 pandemic. |
| 2021 | State final tournament moved back to Champaign, State Farm Center. Tournaments not held due to COVID-19 pandemic. |