Bill Monroe (1911-1996) invented a new style of southern string band music, called bluegrass, in the 1930s and 1940s. Monroe mixed his mountain folk and religious music background with the black country blues that he learned by playing with guitarist Arnold Shultz. His music was remarkable in several ways: the prominence of the mandolin, a very fast tempo, a remarkable vocal technique, the preservation of what Monroe called "old tones," and the wide dissemination of his music through radio and phonograph records.

Monroe had been relegated to playing the mandolin when his brothers claimed the violin and guitar, but his innovative technique gave the instrument a virtuoso part in bluegrass. In addition, instead of adopting the crooning style of country music, Monroe developed mountain vocal technique into a "high, lonesome sound" that perfectly suited the mixture of melancholy and religious subjects that he made into his distinctive repertoire.

Monroe recruited some of the outstanding musicians of his era to play in the Blue Grass Boys, musicians of the caliber of Lester Flatt and Earl Scruggs. The success of his band and bands his musicians created, together with his long career with the Grand Ole Opry, served to make bluegrass a leading form within country and folk music. In the final analysis, it was Monroe's own personality, expressed through his driving mandolin, fast tempos, and evocative songs, that made him such an important part of popular music.