A big band or jazz orchestra is a type of musical ensemble of jazz music that usually consists of ten or more musicians with four sections: saxophones, trumpets, trombones, and a rhythm section. Big bands originated during the early 1910s and dominated jazz in the early 1940s when swing was most popular. The term "big band" is also used to describe a genre of music, although this was not the only style of music played by big bands.

Big bands started as accompaniment for dancing the Lindy Hop. In contrast to the typical jazz emphasis on improvisation, big bands relied on written compositions and arrangements. They gave a greater role to bandleaders, arrangers, and sections of instruments rather than soloists.

Big bands generally have four sections: trumpets, trombones, saxophones, and a rhythm section of guitar, piano, double bass, drums and sometimes vibraphone or other percussion. The division in early big bands, from the 1920s to 1930s, was typically two or three trumpets, one or two trombones, three or four saxophones, and a rhythm section of four instruments. In the 1940s, Stan Kenton's band used up to five trumpets, five trombones (three tenor and two bass trombones), five saxophones (two alto saxophones, two tenor saxophones, one baritone saxophone), and a rhythm section. Duke Ellington at one time used six trumpets. While most big bands dropped the previously common jazz clarinet from their arrangements (other than the clarinet-led orchestras of Artie Shaw and Benny Goodman), many Duke Ellington songs had clarinet parts, often replacing or doubling one of the tenor saxophone parts; more rarely, Ellington would substitute baritone sax for bass clarinet, such as in "Ase's Death" from Swinging Suites. Boyd Raeburn drew from symphony orchestras by adding flute, French horn, strings, and timpani to his band. In the late 1930s, Shep Fields incorporated a solo accordion, temple blocks, piccolo, violins and a viola into his Rippling Rhythm Orchestra. Paul Whiteman also featured a solo accordion in his ensemble.

Jazz ensembles numbering eight (octet), nine (nonet) or ten (tentet) voices are sometimes called "little big bands". During the 1940s, somewhat smaller configurations of the big band emerged in the form of the "rhythm sextet". These ensembles typically featured three or more accordions accompanied by piano, guitar, bass, cello, percussion, and marimba with vibes and were popularized by recording artists such as Charles Magnante, Joe Biviano and John Serry.

Twenty-first century big bands can be considerably larger than their predecessors, exceeding 20 players, with some European bands using 29 instruments and some reaching 50.

In the most common seating for a 17-piece big band, each section is carefully set-up in a way to optimize the band's sound. For the wind players, there are 3 different types of parts: lead parts (including first trumpet, first trombone, and first alto sax), solo parts (including second or fourth trumpet, second trombone, and the first tenor sax), and section members (which include the rest of the band). The band is generally configured so lead parts are seated in the middle of their sections and solo parts are seated closest to the rhythm section. The fourth trombone part is generally played by a bass trombone. In some pieces the trumpets may double on flugelhorn or cornet, and saxophone players frequently double on other woodwinds such as flute, piccolo, clarinet, bass clarinet, or soprano saxophone.

It is useful to distinguish between the roles of composer, arranger and leader. The composer writes original music that will be performed by individuals or groups of various sizes, while the arranger adapts the work of composers in a creative way for a performance or recording. Arrangers frequently notate all or most of the score of a given number, usually referred to as a "chart". Bandleaders are typically performers who assemble musicians to form an ensemble of various sizes, select or create material for them, shape the music's dynamics, phrasing, and expression in rehearsals, and lead the group in performance often while playing alongside them. One of the first prominent big band arrangers was Ferde Grofé, who was hired by Paul Whiteman to write for his “symphonic jazz orchestra”. A number of bandleaders established long-term relationships with certain arrangers, such as the collaboration between leader Count Basie and arranger Neil Hefti. Some bandleaders, such as Guy Lombardo, performed works composed by others (in Lombardo's case, often by his brother Carmen), while others, such as Maria Schneider, take on all three roles. In many cases, however, the distinction between these roles can become blurred. Billy Strayhorn, for example, was a prolific composer and arranger, frequently collaborating with Duke Ellington, but rarely took on the role of bandleader, which was assumed by Ellington, who himself was a composer and arranger.

Typical big band arrangements from the swing era were written in strophic form with the same phrase and chord structure repeated several times. Each iteration, or chorus, commonly follows twelve bar blues form or thirty-two-bar (AABA) song form. The first chorus of an arrangement introduces the melody and is followed by choruses of development. This development may take the form of improvised solos, written solo sections, and "shout choruses".