20th-century classical music was without a dominant style and highly diverse.
At the turn of the century, music was characteristically late Romantic in style. Composers such as Gustav Mahler, Richard Strauss and Jean Sibelius were pushing the bounds of Post-Romantic Symphonic writing. At the same time, the Impressionist movement, spearheaded by Claude Debussy, was being developed in France. The term was actually loathed by Debussy: "I am trying to do 'something different—in a way realities—what the imbeciles call 'impressionism' is a term which is as poorly used as possible, particularly by art critics" —and Maurice Ravel's music, also often labelled with this term, explores music in many styles not always related to it.
Many composers reacted to the Post-Romantic and Impressionist styles and moved in quite different directions. The single most important moment in defining the course of music throughout the century was the widespread break with traditional tonality, effected in diverse ways by different composers in the first decade of the century. From this sprang an unprecedented "linguistic plurality" of styles, techniques, and expression. In Vienna, Arnold Schoenberg developed atonality, out of the expressionism that arose in the early part of the 20th century. He later developed the twelve-tone technique which was developed further by his disciples Alban Berg and Anton Webern; later composers (including Pierre Boulez) developed it further still. Stravinsky (in his last works) explored twelve-tone technique, too, as did many other composers; indeed, even Scott Bradley used the technique in his scores for the Tom and Jerry cartoons.
After the First World War, many composers started returning to the past for inspiration and wrote works that draw elements (form, harmony, melody, structure) from it. This type of music thus became labelled neoclassicism. Igor Stravinsky (Pulcinella and Symphony of Psalms), Sergei Prokofiev (Classical Symphony), Ravel (Le Tombeau de Couperin) and Paul Hindemith (Mathis der Maler) all produced neoclassical works.
Italian composers such as Francesco Balilla Pratella and Luigi Russolo developed musical Futurism. This style often tried to recreate everyday sounds and place them in a "Futurist" context. The "Machine Music" of George Antheil (starting with his Second Sonata, "The Airplane") and Alexander Mosolov (most notoriously his Iron Foundry) developed out of this. The process of extending musical vocabulary by exploring all available tones was pushed further by the use of Microtones in works by Charles Ives, Julián Carrillo, Alois Hába, John Foulds, Ivan Wyschnegradsky, and Mildred Couper among many others. Microtones are those intervals that are smaller than a semitone; human voices and unfretted strings can easily produce them by going in between the "normal" notes, but other instruments will have more difficulty—the piano and organ have no way of producing them at all, aside from retuning and/or major reconstruction.
In the 1940s and 50s composers, notably Pierre Schaeffer, started to explore the application of technology to music in musique concrète. The term Electroacoustic music was later coined to include all forms of music involving magnetic tape, computers, synthesizers, multimedia, and other electronic devices and techniques. Live electronic music uses live electronic sounds within a performance (as opposed to preprocessed sounds that are overdubbed during a performance), Cage's Cartridge Music being an early example. Spectral music (Gerard Grisey and Tristan Murail) is a further development of electroacoustic music that uses analyses of sound spectra to create music. Cage, Berio, Boulez, Milton Babbitt, Luigi Nono and Edgard Varèse all wrote electroacoustic music, often promoted in "happenings".
From the early 1950s onwards, Cage introduced elements of chance into his music. This has resulted in various musical techniques such as indeterminacy, aleatoric music, stochastic music, intuitive music, and free improvisation. Process music (Karlheinz Stockhausen Prozession, Aus den sieben Tagen; and Steve Reich Piano Phase, Clapping Music) explores a particular process which is essentially laid bare in the work. The term Experimental music seems to have been coined by Cage who was interested in writing complete works that performed an unpredictable action, according to the definition "an experimental action is one the outcome of which is not foreseen." The term is also used to describe music within specific genres that pushes against their boundaries or definitions, or else whose approach is a hybrid of disparate styles, or incorporates unorthodox, new, distinctly unique ingredients.
Important cultural trends often informed music of this period, romantic, modernist, neoclassical, postmodernist or otherwise. Igor Stravinsky and Sergei Prokofiev were particularly drawn to primitivism in their early careers, as explored in works such as The Rite of Spring and Chout. Other Russians, notably Dmitri Shostakovich, reflected the social impact of communism and subsequently had to work within the strictures of socialist realism in their music. Other composers, such as Benjamin Britten (War Requiem), explored political themes in their works, albeit entirely at their own volition. Nationalism was also an important means of expression in the early part of the century. The culture of the United States of America, especially, began informing an American vernacular style of classical music, notably in the works of Charles Ives, John Alden Carpenter, and (later) George Gershwin. Folk music (Vaughan Williams' Variants on Dives and Lazarus, Gustav Holst's A Somerset Rhapsody) and Jazz (Gershwin, Leonard Bernstein, Darius Milhaud's La création du monde) were also influential.
In the latter quarter of the century, eclecticism and polystylism became important. These, as well as minimalism, New Complexity, and New Simplicity, are more fully explored in their respective articles. The term postmodern music is often applied to music that "reacts" to Modernism, though it is not always clear what the "reaction" is. .