structural:scholar_edition
Late-Romantic, modernist and postmodernist eras

Towards the end of the 19th century, Gustav Mahler began writing long, large-scale symphonies that he continued composing into the early 20th century. His Third Symphony, completed in 1896, is one of the longest regularly performed symphonies at around 100 minutes in length for most performances. The Eighth Symphony was composed in 1906 and is nicknamed the "Symphony of a Thousand" because of the large number of voices required to perform the work.

The 20th century saw further diversification in the style and content of works that composers labeled symphonies. Some composers, including Dmitri Shostakovich, Sergei Rachmaninoff, and Carl Nielsen, continued to write in the traditional four-movement form, while other composers took different approaches: Jean Sibelius ' Symphony No. 7, his last, is in one movement, Richard Strauss ' Alpine Symphony, in one movement, split into twenty-two parts, detailing an eleven hour hike through the mountains and Alan Hovhaness 's Symphony No. 9, Saint Vartan —originally Op. 80, changed to Op. 180—composed in 1949–50, is in twenty-four.

A concern with unification of the traditional four-movement symphony into a single, subsuming formal conception had emerged in the late 19th century. This has been called a "two-dimensional symphonic form", and finds its key turning point in Arnold Schoenberg 's Chamber Symphony No. 1, Op. 9 (1909), which was followed in the 1920s by other notable single-movement German symphonies, including Kurt Weill 's First Symphony (1921), Max Butting 's Chamber Symphony, Op. 25 (1923), and Paul Dessau 's 1926 Symphony.

Alongside this experimentation, other 20th-century symphonies deliberately attempted to evoke the 18th-century origins of the genre, in terms of form and even musical style, with prominent examples being Sergei Prokofiev 's Symphony No. 1 "Classical" of 1916–17 and the Symphony in C by Igor Stravinsky of 1938–40.

There remained, however, certain tendencies. Designating a work a "symphony" still implied a degree of sophistication and seriousness of purpose. The word sinfonietta came into use to designate a work that is shorter, of more modest aims, or "lighter" than a symphony, such as Sergei Prokofiev 's Sinfonietta for orchestra.

In the first half of the century, composers including Edward Elgar, Gustav Mahler, Jean Sibelius, Carl Nielsen, Igor Stravinsky, Bohuslav Martinů, Roger Sessions, Sergei Prokofiev, Rued Langgaard and Dmitri Shostakovich composed symphonies "extraordinary in scope, richness, originality, and urgency of expression". One measure of the significance of a symphony is the degree to which it reflects conceptions of temporal form particular to the age in which it was created. Five composers from across the span of the 20th century who fulfil this measure are Jean Sibelius, Igor Stravinsky, Luciano Berio (in his Sinfonia, 1968–69), Elliott Carter (in his Symphony of Three Orchestras, 1976), and Pelle Gudmundsen-Holmgreen (in Symphony/Antiphony, 1980).

From the mid-20th century into the 21st there has been a resurgence of interest in the symphony with many postmodernist composers adding substantially to the canon, not least in the United Kingdom: Peter Maxwell Davies (10), Robin Holloway (1), David Matthews (9), James MacMillan (5), Peter Seabourne (6), and Philip Sawyers (6). British composer Derek Bourgeois has surpassed the number of symphonies written by Haydn, with 116 symphonies. The greatest number of symphonies to date has been composed by the Finn Leif Segerstam, whose list of works includes 371 symphonies.