A form of American roots or country music which has its own roots in Irish, Scottish and English traditional music.

Bluegrass was inspired by the music of immigrants from the British Isles (particularly the Scots-Irish immigrants of Appalachia), as well as that of rural African-Americans, jazz, and blues. In bluegrass, as in jazz, each instrument takes a turn playing the melody and improvising around it, while the others revert to backing. This is in contrast to old-time music, in which all instruments play the melody together or one instrument carries the lead throughout while the others provide accompaniment.

Bluegrass is distinctively acoustic, rarely using electric instruments. The fiddle, banjo, acoustic guitar, mandolin and upright bass are sometimes joined by the resonator guitar (popularly known by the Dobro brand name). This instrumentation originated in rural black dance bands and was being abandoned by those groups (in favour of blues and jazz ensembles) when picked up by white musicians. Instrumental solos are improvised and can frequently be technically demanding.

Besides instrumentation, a distinguishing characteristic of bluegrass is vocal harmony featuring 2, 3, or 4 parts, often featuring a dissonant or modal sound in the highest voice. This vocal style has been characterised as the 'high lonesome sound'.

The topical and narrative themes of many bluegrass songs are highly reminiscent of folk music. In fact, many songs that are widely considered to be bluegrass are older works legitimately classified as folk or old-time performed in a bluegrass style. However, from a musical genre perspective, bluegrass is more appropriately classified as a form of country music, sharing as it does much of the same Appalachian culture, instrumentation and emphasis on traditional songs, often with sentimental or religious themes.

Bluegrass as a style developed during the mid 1940s, but there are few recordings from this time due to war rationing. Bill Monroe is often referred to as the founding father of bluegrass music, since the style was named after his band, the Blue Grass Boys, formed in 1939. The 1945 addition of banjo player Earl Scruggs, who played with a 3-finger roll originally developed by Snuffy Jenkins but now almost universally known as 'Scruggs style', is pointed to as the key moment in the development of this genre. Monroe's 1945-48 band created the definitive sound and instrumental configuration that remains a model to this day.

Bluegrass became a genre as other artists such as the Stanley Brothers and Flatt and Scruggs recorded music in this style from the late 1940s.

The next generation of bluegrass artists came to prominence in the mid- to late-1960s, including the Dillards, Norman Blake and Tony Rice. With the second generation came a growth in progressive bluegrass, as exemplified by bands such as the the Country Gentlemen. In that vein, first-generation bluegrass fiddler Vassar Clements, mandolin virtuoso David Grisman, Grateful Dead frontman Jerry Garcia (on banjo) and Peter Rowan as lead vocalist collaborated on the album Old and in the Way and helped to expose progressive bluegrass to a rock music audience.

In the late 1990s, several mainstream country musicians recorded bluegrass albums. Ricky Skaggs, who began as a bluegrass musician and crossed over to mainstream country in the 1980s, returned to bluegrass in 1996, and since then has recorded several bluegrass albums and tours with his bluegrass band Kentucky Thunder. Around the same time, country music superstar Dolly Parton has released several bluegrass albums. Also, although often considered a crossover or mainstream country artist, no discussion of recent developments in bluegrass music would be complete without mention of Alison Krauss, a vocalist/fiddler who with her band, Union Station, has released numerous albums with a strong bluegrass element.