The Illinois High School Boys Basketball Championship is a single elimination tournament held each spring in the United States. It is organized by the Illinois High School Association (IHSA).
From 1908 to 1971, it was a single tournament contested by nearly all high schools in Illinois. In 1972, the tournament was divided into two divisions based on school size, (A and AA), each producing a separate champion. In 2008, the tournament was divided into four separate divisions (1A, 2A, 3A, and 4A being the larger schools). The Illinois High School Basketball Championship was the first tournament to be called 'March Madness'. The term was first used about the Illinois tournament in 1939, decades before it was used about NCAA basketball tournament.