Glen Leon Dallin Jr. (March 26, 1918 – December 31, 1993) was an influential American music theorist known for his significant contributions to the understanding of twentieth-century music composition. Born in Silver City, Utah, Dallin pursued his passion for music by obtaining both his bachelor's and master's degrees from the Eastman School of Music, followed by a Ph.D. from the University of California, Berkeley.

Dallin's academic career included teaching positions at Brigham Young University from 1948 to 1955 and at California State University, Long Beach from 1955 until his retirement in 1983. His most notable work, "Techniques of Twentieth Century Composition: A Guide to the Materials of Modern Music," published in 1957 and revised in 1964 and 1974, became a cornerstone in the field of music theory, particularly before the advent of set theory in the 1970s.

In addition to his theoretical work, Dallin was a prolific composer, with a collection that includes orchestral, band, choral, chamber, and solo instrumental music, as well as film and theater scores. His legacy is preserved in the Leon Dallin Collection at the Sibley Music Library, Eastman School of Music, which houses a comprehensive archive of his compositions and related materials.