The Interstate Intercollegiate Athletic Conference (IIAC) was a college athletic conference that existed from 1908 to 1970 in the United States.

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At one time the Illinois Intercollegiate Athletic Conference, or IIAC, was a robust league that claimed most of the Illinois institutions of higher education. It was nicknamed the "Little Nineteen," but in 1928 had a membership of 23 schools. Former Illinois State University track coach Joseph Cogdal, associated with the IIAC for 43 years of its 62-year history, noted that the league had roots in the 1870s when a number of schools banded together for oratorical contests. Their first intercollegiate football game was played in 1881 between Illinois State University and Knox College, and by 1894 a football association was established.

The IIAC was formed in April 1908 with eight charter members: Illinois State Normal University (now Illinois State University), Illinois Wesleyan University, Bradley Polytechnic Institute (now Bradley University), Millikin University, Monmouth College, Knox College, Lombard College and Illinois College. The first track meet was held on May 22, 1908. The group quickly expanded. Eastern Illinois State Teachers College (now Eastern Illinois University) and Western Illinois University joined in 1912 and 1914 respectively.

In 1920, the name "Illinois Intercollegiate Athletic Conference" was adopted, providing the initials IIAC. Conference membership reached a peak of 23 member schools in 1928, when virtually all of the small colleges in Illinois were included.

Private schools withdrew during much of the 1930s, until in 1942 only the five state schools remained: Illinois State University, Eastern Illinois University, Northern Illinois University, Southern Illinois University Carbondale and Western Illinois University. In 1950, the league name became the Interstate Intercollegiate Athletic Conference, when Central Michigan University and Eastern Michigan University brought the membership to seven. In 1961-62, Eastern Michigan University and Southern Illinois University Carbondale withdrew; Northern Illinois University followed in 1965-66. The conference disbanded at the end of the 1969–70 academic year.

• List of Interstate Intercollegiate Athletic Conference football standings
• List of defunct college football conferences
• Mid-American Conference
• Missouri Valley Football Conference
• Ohio Valley Conference
• College Conference of Illinois and Wisconsin