20th century classical music begins with the late Romantic style of Sergei Rachmaninoff, Impressionism of Claude Debussy and Maurice Ravel, American Vernacular music of Charles Ives and George Gershwin, and continues through the Neoclassicism of middle-period Igor Stravinsky, the twelve-tone music of Arnold Schoenberg, Alban Berg, and Anton Webern. It ranges to such distant sound-worlds as the total serialism of Pierre Boulez, the simple harmonies and rhythms of minimalist composers such as Steve Reich, and Philip Glass, the musique concrète of Pierre Schaeffer and Pierre Henry, the microtonal music written by Easley Blackwood, Alois Hába, Ben Johnston, Harry Partch, and others, the aleatoric music of John Cage, the Intuitive music of Karlheinz Stockhausen, and the polystylism of Alfred Schnittke.

Perhaps the most salient feature during this time period of classical music was the increased use of dissonance. Because of this, the twentieth century is sometimes called the "Dissonant Period" of classical music, which followed the common practice period, which emphasized consonance. The watershed transitional moment was the international Paris Exposition celebrating the centennial of the French Revolution. This exposition brought a variety of non-Western performing artists to Paris, influencing Debussy and Mahler in particular. While some writers hold that Debussy's Prélude à l'après-midi- d'un faune and Schoenberg's Verklärte Nacht are dramatic departures from Romanticism and have strong modernist traits (Schwartz and Godfrey 1993, 9–43), others hold that the Schoenberg work is squarely within the late-Romantic tradition of Wagner and Brahms (Neighbour 2001, 582).

Among the most prominent composers of the twentieth century were Béla Bartók, Gustav Mahler, Richard Strauss, Giacomo Puccini, Claude Debussy, Maurice Ravel, Charles Ives, Edward Elgar, Frederick Delius, Arnold Schoenberg, Jean Sibelius, Elliott Carter, Sergei Rachmaninoff, Sergei Prokofiev, Gabriel Fauré, Alberto Ginastera, Gian-Carlo Menotti, Igor Stravinsky, Dmitri Shostakovich, Alban Berg, Manuel de Falla, Peter Maxwell Davies, Ottorino Respighi, John Cage, Benjamin Britten, Anton Webern, Ralph Vaughan Williams, Aaron Copland, Carl Nielsen, Paul Hindemith, György Ligeti, Olivier Messiaen, Kurt Weill, Milton Babbitt, Samuel Barber, Luciano Berio, Karlheinz Stockhausen, Heitor Villa-Lobos, Henri Dutilleux and Witold Lutosławski. Classical music also had a significant cross fertilization with jazz, with several composers being able to work in both genres, including George Gershwin and Leonard Bernstein.

An important feature of twentieth-century concert music is the splitting of the audience into traditional and avant-garde, with many figures prominent in one world considered minor or unacceptable in the other. Composers such as Anton Webern, Elliott Carter, Edgard Varèse, Milton Babbitt, and Luciano Berio have devoted followings within the avant-garde, but are often attacked outside of it. As time has passed, however, it is increasingly accepted, though by no means universally so, that the boundaries are more porous than the many polemics would lead one to believe: many of the techniques pioneered by the above composers show up in popular music by The Beatles, Deep Purple, Yes, Genesis, King Crimson, Pink Floyd, ELP, Mike Oldfield, Enigma, Vangelis, Jean Michel Jarre and in film scores that draw mass audiences.

It should be kept in mind that this article presents an overview of twentieth-century classical music and many of the composers listed under the following trends and movements may not identify exclusively as such and may be considered as participating in different movements. For instance, at different times during his career, Igor Stravinsky may be considered a romantic, modernist, neoclassicist, and a serialist.

The twentieth century was also an age where recording and broadcast changed the economics and social relationships inherent in music. An individual in the 19th century made most music themselves, or attended performances. An individual in the industrialized world had access to radio, television, phonograph and later digital music such as the CD.