Marching Illini (MI) is the name of the marching band of the University of Illinois at Urbana-Champaign. Considered to be in the top tier of American university college marching bands, the Marching Illini claim to have originated many marching band and university traditions such as the school mascot and Homecoming.

The band primarily performs before, during, and after University of Illinois home football games. The band also performs an indoor concert at the Assembly Hall featuring special lighting effects, performances by individual sections, and slightly pithy comic routines. Other performances include a drill for the Illini Marching Festival, halftime performances at infrequent postseason bowl games, and an annual away performance at the home football game of another Big Ten school. The MI do not typically perform for parades other than the annual Homecoming parade and when marching to and from Memorial Stadium.

The band performs in a style common to other marching bands of the Big Ten collegiate athletic conference. While the band prides itself on developing innovations in the marching band field, their style is somewhat conservative when compared to other marching bands. The band moves between precise drill formations (unlike East Coast scramble bands) and usually remains in a symmetric arrangement about the 50-yard line in an abstract form (in contrast drum and bugle corps who typically have a much larger variety of formations). The drill style of the band is a necessity since the band performs an entirely new show for every home football game; thus the formations, while still considerably complex, must also be quickly learned by the ensemble.

Instrumentation of the Marching Illini is based upon the composition of a typical concert band, but modified in several ways to support outdoor performance. Piccolos are used instead of Flutes. Mellophones, a valved brass instrument in the same range as a French Horn, add an alto voice to the brass section. A larger-than-usual section of metal Sousaphones adds a deeper low brass sound, not to mention visual interest. The higher than usual proportion of low- and mid-range brass instruments (baritone, trombone, sousaphone, and mellophone) gives the band a unique rich, full sound among marching bands, while the complete woodwind section allows the band to play traditional concert band repertoire (unlike brass-only marching bands). An auxiliary flag corps and female dance squad (named "Illinettes") add a further visual element to the band's performances. Approximately 288 of the band's 300+ members perform on the football field during each show.

The band is perhaps most well known for its traditional Three in One performance at the conclusion of every halftime show. The focus of the performance is the University of Illinois school symbol, Chief Illiniwek, a student-dancer dressed in a Native American costume who is commonly confused as the school's mascot. Chief Illiniwek performs a stylized Native American Fancy Dance while the band plays a medley of two traditional Illinois-themed marches plus the school's Alma Mater chorale. The tradition has drawn a significant amount of controversy from Native American groups who say that the performance is racially insensitive to Native Americans. However, supporters of the tradition respond by saying that the dance is presented in a dignified, honorable manner not intended to draw offense. The school's Board of Trustees discusses the issue from time to time but has yet to take substantive action in either way in recent years. The performance begins when the band rearranges itself into a large Block I formation and beings to march downfield while singing the lyrics to "The March of the Illini." When the band reaches the North endzone, Chief Illiniwek emerges from hiding within the band and begins an athletic dance across the length of the football field. The band then performs a drill where the letters I L L I N I materialize in succession from the massed band and begin to march toward the center of the field. At the midpoint of "The Pride of the Illini," the Chief stops his dance in the center of the field and raises his arms, prompting the spectators to do likewise. Everyone in the stadium then sings a school loyalty song named "Hail to the Orange." At the conclusion of the singing, the Chief performs another fancy dance routine as the band plays the last refrain of "Pride of the Illini" at an increasing tempo.

The present controversy, however, should not overshadow the band's outstanding history and performance caliber. In recognition of this, the Marching Illini was awarded the prestigious Louis Sudler Intercollegiate Marching Band Trophy in 1983, the second band to receive the award. The band also claims to have first performed and/or originated many features now common to today's marching bands. The MI performed the first halftime show in 1907, made the first school letter formation on a field, invented the concept of school mascot (derived from the School's symbol Chief Illiniwek), was the first band to sing a cappella on a football field, initiated the first student card cheering section ("Block I"), recorded the first marching band compact disc, and was the first college band to use bugles and mallets in a field show.