Originating in vocal jazz, scat singing or scatting is vocal improvisation with wordless vocables, nonsense syllables or without words at all. In scat singing, the singer improvises melodies and rhythms using the voice solely as an instrument rather than a speaking medium. This is different from vocalese, which uses recognizable lyrics that are sung to pre-existing instrumental solos.

Though scat singing is improvised, the melodic lines are often variations on scale and arpeggio fragments, stock patterns and riffs, as is the case with instrumental improvisers. As well, scatting usually incorporates musical structure. All of Ella Fitzgerald's scat performances of "How High the Moon", for instance, use the same tempo, begin with a chorus of a straight reading of the lyric, move to a "specialty chorus" introducing the scat chorus, and then the scat itself. Will Friedwald has compared Ella Fitzgerald to Chuck Jones directing his Roadrunner cartoon—each uses predetermined formulas in innovative ways.

The deliberate choice of scat syllables is also a key element in vocal jazz improvisation. Syllable choice influences the pitch, articulation, coloration, and resonance of the performance. Syllable choice also differentiated jazz singers' personal styles: Betty Carter was inclined to use sounds like "louie-ooie-la-la-la" (soft-tongued sounds or liquids) while Sarah Vaughan would prefer "shoo-doo-shoo-bee-ooo-bee" (fricatives, plosives, and open vowels). The choice of scat syllables can also be used to reflect the sounds of different instruments. The comparison of the scatting styles of Ella Fitzgerald and Sarah Vaughan reveals that Fitzgerald's improvisation mimics the sounds of swing-era big bands with which she performed, while Vaughan's mimics that of her accompanying bop-era small combos.

Humor is another important element of scat singing. Bandleader Cab Calloway exemplified the use of humorous scatting. Other examples of humorous scatting include Slim Gaillard, Leo Watson, and Bam Brown's 1945 song "Avocado Seed Soup Symphony," in which the singers scat variations on the word "avocado" for much of the recording.

In addition to such nonsensical uses of language, humor is communicated in scat singing through the use of musical quotation. Leo Watson, who performed before the canon of American popular music, frequently drew on nursery rhymes in his scatting. This is called using a compression. Similarly, Ella Fitzgerald's scatting, for example, drew extensively on popular music. In her 1960 recording of "How High the Moon" live in Berlin, she quotes over a dozen songs, including "The Peanut Vendor," "Heat Wave," "A-Tisket, A-Tasket," and "Smoke Gets in Your Eyes."

Improvisational singing of nonsense syllables occurs in many cultures, such as diddling or lilting in Ireland, German yodeling, Sámi joik, and speaking in tongues in various religious traditions.

Although Louis Armstrong's 1926 recording of "Heebie Jeebies" is often cited as the first modern song to employ scatting, there are many earlier examples. One early master of ragtime scat singing was Gene Greene who recorded scat choruses in his song "King of the Bungaloos" and several others between 1911 and 1917. Entertainer Al Jolson scatted through a few bars in the middle of his 1911 recording of "That Haunting Melody." Gene Greene's 1917 "From Here to Shanghai," which featured faux-Chinese scatting, and Gene Rodemich's 1924 "Scissor Grinder Joe" and "Some of These Days" also pre-date Armstrong. Cliff "Ukulele Ike" Edwards scatted an interlude on his 1923 "Old Fashioned Love" in lieu of using an instrumental soloist. One of the early female singers to use scat was Aileen Stanley, who included it at the end of a duet with Billy Murray in their hit 1924 recording of "It Had To Be You" (Victor 19373).

Jazz pianist Jelly Roll Morton credited Joe Sims of Vicksburg, Mississippi, as the creator of scat around the turn of the 20th century. In a conversation between Alan Lomax and Jelly Roll Morton, Morton recounted the history of scat: