Atonality in its broadest sense is music that lacks a tonal center, or key. Atonality, in this sense, usually describes compositions written from about the early 20th century to the present day, where a hierarchy of harmonies focusing on a single, central triad is not used, and the notes of the chromatic scale function independently of one another. More narrowly, the term atonality describes music that does not conform to the system of tonal hierarchies that characterized European classical music between the seventeenth and nineteenth centuries. "The repertory of atonal music is characterized by the occurrence of pitches in novel combinations, as well as by the occurrence of familiar pitch combinations in unfamiliar environments".

The term is also occasionally used to describe music that is neither tonal nor serial, especially the pre-twelve-tone music of the Second Viennese School, principally Alban Berg, Arnold Schoenberg, and Anton Webern. However, "as a categorical label, 'atonal' generally means only that the piece is in the Western tradition and is not 'tonal'", although there are longer periods, e.g., medieval, renaissance, and modern modal music to which this definition does not apply. "Serialism arose partly as a means of organizing more coherently the relations used in the pre-serial 'free atonal' music. ... Thus, many useful and crucial insights about even strictly serial music depend only on such basic atonal theory".

Late 19th- and early 20th-century composers such as Alexander Scriabin, Claude Debussy, Paul Hindemith, Béla Bartók, Sergei Prokofiev, Igor Stravinsky, and Edgard Varèse, have written music that has been described, in full or in part, as atonal.^{[excessive citations]}

While music without a tonal center had been previously written, for example Franz Liszt's Bagatelle sans tonalité of 1885, it is with the coming of the twentieth century that the term atonality began to be applied to pieces, particularly those written by Arnold Schoenberg and The Second Viennese School. The term "atonality" was coined in 1907 by Joseph Marx in a scholarly study of tonality, which was later expanded into his doctoral thesis.

Their music arose from what was described as the "crisis of tonality" between the late nineteenth century and early twentieth century in classical music. This situation had arisen over the course of the nineteenth century due to the increasing use of

ambiguous chords, improbable harmonic inflections, and more unusual melodic and rhythmic inflections than what was possible within the styles of tonal music. The distinction between the exceptional and the normal became more and more blurred. As a result, there was a "concomitant loosening" of the synthetic bonds through which tones and harmonies had been related to one another. The connections between harmonies were uncertain even on the lowest chord-to-chord level. On higher levels, long-range harmonic relationships and implications became so tenuous, that they hardly functioned at all. At best, the felt probabilities of the style system had become obscure. At worst, they were approaching a uniformity, which provided few guides for either composition or listening.

The first phase, known as "free atonality" or "free chromaticism", involved a conscious attempt to avoid traditional diatonic harmony. Works of this period include the opera Wozzeck (1917–1922) by Alban Berg and Pierrot lunaire (1912) by Schoenberg.

The second phase, begun after World War I, was exemplified by attempts to create a systematic means of composing without tonality, most famously the method of composing with 12 tones or the twelve-tone technique. This period included Berg's Lulu and Lyric Suite, Schoenberg's Piano Concerto, his oratorio Die Jakobsleiter and numerous smaller pieces, as well as his last two string quartets. Schoenberg was the major innovator of the system. His student, Anton Webern, however, is anecdotally claimed to have begun linking dynamics and tone color to the primary row, making rows not only of pitches but of other aspects of music as well. However, actual analysis of Webern's twelve-tone works has so far failed to demonstrate the truth of this assertion. One analyst concluded, following a minute examination of the Piano Variations, op. 27, that