Bluegrass music is a genre of American roots music that developed in the 1940s in the Appalachian region of the United States. The genre derives its name from the band Bill Monroe and the Blue Grass Boys. Bluegrass has roots in African American genres like blues and jazz and North European genres, such as Irish ballads and dance tunes. Unlike country, it is traditionally played exclusively on acoustic instruments such as the fiddle, mandolin, banjo, guitar and upright bass. It was further developed by musicians who played with Monroe, including 5-string banjo player Earl Scruggs and guitarist Lester Flatt. Bill Monroe once described bluegrass music as, "It's a part of Methodist, Holiness and Baptist traditions. It's blues and jazz, and it has a high lonesome sound."

Bluegrass features acoustic stringed instruments and emphasizes the off-beat. The off-beat can be "driven" (played close to the previous bass note) or "swung" (played farther from the previous bass note). Notes are anticipated, in contrast to laid-back blues where notes are behind the beat; this creates the higher energy characteristic of bluegrass. In bluegrass, as in most forms of jazz, one or more instrumentalists take a turn playing the melody and improvising around it, while the others perform accompaniment; this is especially typified in tunes called breakdowns. This is in contrast to old-time music, where all instrumentalists play the melody together, or one instrument carries the lead throughout while the others provide accompaniment. Breakdowns are often characterized by rapid tempos and unusual instrumental dexterity, and sometimes by complex chord changes.

The violin (also known as the fiddle), five-string banjo, guitar, mandolin, and upright bass (string bass) are often joined by the resonator guitar (also referred to as a Dobro) and (occasionally) harmonica or Jew's harp. This instrumentation originated in rural dance bands and is the basis on which the earliest bluegrass bands were formed.

The fiddle, made by Italians and first used in sixteenth century Europe, was one of the first instruments to be brought into America. It became popular due to its small size and versatility. Fiddles are also used in country, classical, cajun, and old time music.

Banjos were brought to America through the African slave trade. They began receiving attention from white Americans when minstrel shows incorporated the banjo as part of their acts. The "clawhammer", or two finger style playing, was popular before the Civil War. Now, however, banjo players use mainly the three-finger picking style made popular by banjoists such as Earl Scruggs.

Guitars are used primarily for rhythmic purposes. Other instruments may provide a solo on top of the guitar during breaks, guitarists may also provide these solos on occasion. The instrument originates from eighteenth century Spain, but there were no American-made models until the C.F. Martin Company started to manufacture them in the 1830s. The guitar is now most commonly played with a style referred to as flatpicking, unlike the style of early bluegrass guitarists such as Lester Flatt, who used a thumb pick and finger pick.

Bassists almost always play pizzicato, occasionally adopting the "slap-style" to accentuate the beat. A bluegrass bass line is generally a rhythmic alternation between the root and fifth of each chord, with occasional walking bass excursions.

Instrumentation has been a continuing topic of debate. Traditional bluegrass performers believe the "correct" instrumentation is that used by Bill Monroe's band, the Blue Grass Boys (guitar, mandolin, fiddle, banjo, and bass). Departures from the traditional instrumentation have included dobro, accordion, harmonica, piano, autoharp, drums, electric guitar, and electric versions of other common bluegrass instruments, resulting in what has been referred to as "new grass." Despite this debate, even Monroe himself was known to experiment with instrumentation; he once even used a string orchestra, choir, and pre-recorded bird-song track.