Bebop or bop is a form of jazz characterized by fast tempos and improvisation based on the combination of harmonic structure and melody. It was developed in the early and mid-1940s. It first surfaced in musicians' argot some time during the first two years of the Second World War.

The 1939 recording of "Body and Soul" by Coleman Hawkins is an important antecedent of bebop. Hawkins' willingness to stray — even briefly — from the ordinary resolution of musical themes and his playful jumps to double-time signaled a departure from existing jazz. The recording was popular; but more importantly, from a historical perspective, Hawkins became an inspiration to a younger generation of jazz musicians, most notably Charlie Parker, in Kansas City.

In the 1940s, the younger generation of jazz musicians forged a new style out of the swing music of the 1930s. Mavericks like Dizzy Gillespie, Charlie Parker, Bud Powell and Thelonious Monk were influenced by the preceding generation's adventurous soloists, such as pianists Art Tatum and Earl Hines, tenor saxophonists Coleman Hawkins and Lester Young and trumpeter Roy Eldridge. Gillespie and Parker, both out of the Earl Hines Band in Chicago had traveled with some of the pre-bop masters, including Jack Teagarden, Earl Hines and Jay McShann. These forerunners of bebop began exploring advanced harmonies, complex syncopation, altered chords, and chord substitutions and the bop generation advanced these techniques with a more freewheeling and often arcane approach.

Minton's Playhouse in New York served as a workout room and experimental theater for early bebop players, including Charlie Christian, who had already hinted at the bop style in innovative solos with Benny Goodman's band.

Christian's major influence was in the realm of rhythmic phrasing. Christian commonly emphasized weak beats and off beats, and often ended his phrases on the second half of the fourth beat. Christian experimented with asymmetrical phrasing, which was to become a core element of the new bop style. Swing improvisation was commonly constructed in two or four bar phrases that corresponded to the harmonic cadences of the underlying song form. Bop improvisers would often deploy phrases over an odd number of bars, and overlap their phrases across bar lines and across major harmonic cadences. Christian and the other early boppers would also begin stating a harmony in their improvised line before it appeared in the song form being outlined by the rhythm section. This momentary dissonance creates a strong sense of forward motion in the improvisation. Swing improvisers commonly emphasized the first and third beats of a measure. But in a bebop composition such as Dizzy Gillespie's "Salt Peanuts", the rhythmic emphasis switches to the second and fourth beats of the measure. Such new rhythmic phrasing techniques give the typical bop solo a feeling of floating free over the underlying song form, rather than being tied into the song form.

Swing drummers had kept up a steady four-to-the-bar pulse on the bass drum. Bop drummers, led by Kenny Clarke, moved the drumset's time-keeping function to the ride or hi-hat cymbal, reserving the bass drum for accents. Bass drum accents were colloquially termed "dropping bombs." Notable bop drummers such as Max Roach, Philly Joe Jones, Roy Haynes, and Kenny Clarke began to support and respond to soloists, almost like a shifting call and response.

This change increased the importance of the string bass. Now, the bass not only maintained the music's harmonic foundation, but also became responsible for establishing a metronomic rhythmic foundation by playing a "walking" bass line of four quarter notes to the bar. While small swing ensembles commonly functioned without a bassist, the new bop style required a bass in every small ensemble.

By 1950, a second wave of bebop musicians — such as Clifford Brown, Sonny Stitt, and Fats Navarro — began to smooth out the rhythmic eccentricities of early bebop. Instead of using jagged phrasing to create rhythmic interest, as the early boppers had, these musicians constructed their improvised lines out of long strings of eighth notes, and simply accented certain notes in the line to create rhythmic variety.