Klezmer (Yiddish: קלעזמער or כּלי־זמר) is an instrumental musical tradition of the Ashkenazi Jews of Central and Eastern Europe. The essential elements of the tradition include dance tunes, ritual melodies, and virtuosic improvisations played for listening; these would have been played at weddings and other social functions. The musical genre incorporated elements of many other musical genres including Ottoman (especially Greek and Romanian) music, Baroque music, German and Slavic folk dances, and religious Jewish music. As the music arrived in the United States, it lost some of its traditional ritual elements and adopted elements of American big band and popular music. Among the European-born klezmers who popularized the genre in the United States in the 1910s and 1920s were Dave Tarras and Naftule Brandwein; they were followed by American-born musicians such as Max Epstein, Sid Beckerman and Ray Musiker.

After the destruction of Jewish life in Eastern Europe during the Holocaust, and a general fall in the popularity of klezmer music in the United States, the music began to be popularized again in the late 1970s in the so-called Klezmer Revival. During the 1980s and onwards, musicians experimented with traditional and experimental forms of the genre, releasing fusion albums combining the genre with jazz, punk, and other styles. By the 1980s and 1990s the American revival spread to Europe and inspired a new interest in the genre in places such as Germany, France, Poland and Russia. A parallel tradition has also continued in Israel with such figures as Moussa Berlin.

The term klezmer, as used in the Yiddish language, has a Hebrew etymology: klei, meaning "tools, utensils or instruments of" and zemer, "melody"; leading to k'lei zemer כְּלֵי זֶמֶר‎, meaning "musical instruments". Over time the usage of "klezmer" in a Yiddish context evolved to describe musicians instead of their instruments, first in Bohemia in the second half of the sixteenth century and then in Poland, possibly as a response to the new status of the musicians who were at that time forming professional guilds. Previously the musician may have been referred to as a lets (לץ) or other terms. After the term klezmer became the preferred term for these professional musicians in Yiddish-speaking Eastern Europe, other types of musicians were more commonly known as muziker or muzikant. Twentieth century Russian scholars sometimes used the term klezmer; Ivan Lipaev did not use it, but Moisei Beregovsky did when publishing in Yiddish or Ukrainian.

It was not until the late 20th century that the word "klezmer" became a commonly known English-language term. During that time, through metonymy it came to refer not only to the musician but to the musical genre they played, a meaning which it had not had in Yiddish. Early 20th century recording industry materials and other writings had referred to it as Hebrew, Jewish, or Yiddish dance music, or sometimes using the Yiddish term Freilech music ("Cheerful music"). The term 'klezmer' to refer to a genre of music was popularized as a marketing term in the late 1970s by Revival bands; Walter Zev Feldman, whose 1979 LP with Andy Statman used the term, claims credit for this shift in usage.

The traditional style of playing klezmer music, including tone, typical cadences, and ornamentation, sets it apart from other genres. Although klezmer music emerged from a larger Eastern European Jewish musical culture that included Jewish cantorial music, Hasidic Niguns, and later Yiddish theatre music, it also borrowed from the surrounding folk musics of Central and Eastern Europe and from cosmopolitan European musical forms. Therefore it evolved into an overall style which has recognizable elements from all of those other genres.

Few klezmer musicians before the late nineteenth century had conservatory musical training, but they generally learned through apprenticeship and inherited a rich tradition with its own advanced musical techniques. Each musician had their understanding of how the style should be "correctly" performed. The usage of these ornaments was not random; the matters of "taste", self-expression, variation and restraint were and remain important elements of how to interpret the music.

Klezmer musicians apply the overall style to available specific techniques on each melodic instrument. They incorporate and elaborate the vocal melodies of Jewish religious practice, including the vocal style of the Hazzan, Jewish prayer, and paraliturgical song, extending the range of human voice into the musical expression possible on instruments. Among those stylistic elements that are considered typically "Jewish" in klezmer music are those which are shared with cantorial or Hasidic vocal ornaments, including imitations of sighing or laughing. Various Yiddish terms were used for these vocal-like ornaments such as קרעכץ (Krekhts, "groan" or "moan"), קנײטש (kneytsh, "wrinkle" or "fold"), and קװעטש (kvetsh, "pressure" or "stress"). Other ornaments such as trills, grace notes, appoggiaturas, glitshn (glissandos), tshoks (a kind of bent notes of cackle-like sound), flageolets (string harmonics), pedal notes, mordents, slides and typical klezmer cadences are also important to the style. In particular, the cadences which draw on religious Jewish music identify a piece more strongly as a klezmer tune, even if its broader structure was borrowed from a non-Jewish source. Sometimes the term dreydlekh is used only for trills, while other use it for all klezmer ornaments. Unlike in Classical music, vibrato is used sparingly, and is treated as another type of ornament.

The accompaniment style varies a lot depending on instrumentation and context, ranging from playing in octaves without harmonization, to partial chords played by a second violinist, to very elaborate harmonized brass bands arrangements in the twentieth century.