Symphony, Op. 21 was composed by Anton Webern between 1927 and 1928. It was his first twelve-tone orchestral work. The two-movement work lasts 10–20 minutes and contains Alpine topics, abstraction, and intricate musical form, including some fixed register. The Symphony was influenced by Gustav Mahler. Alexander Smallens conducted the world premiere at New York's Town Hall on 18 December 1929.

Webern was an alpinist who enjoyed mountain excursions. He loved the quiet otherworldliness of the altitude. He referred to these landscapes as "up there", a spiritual, utopian realm. He continued Mahler's practice of portraying Alpine stillness and spaciousness in his music.

His favorite mountain was the Schneealpe. Webern climbed it twice in 1928 while he was writing the Symphony, summiting only once in July. He also climbed the Hochschwab twice while he was composing the work.

After finishing the Symphony, Webern wrote to the poet Hildegard Jone on 6 August 1928. Replying to her suggestion that "progress is made only inwards", he concurred, "I understand 'Art' to mean the faculty of bringing a thought into the clearest, simplest, i.e. 'most comprehensible' form." Pointing to previous masters like Beethoven, he continued, "...I have always only endeavoured to do exactly as they did: to represent as clearly as possible that which is given to me to say."

Webern's Symphony is scored for clarinet, bass clarinet, 2 horns, harp, and strings without basses. He was paring down his orchestration compared to his earlier work. In the same period, he was reducing the "extravagant" wind parts in Sechs Stücke, Op. 6. In the summer of 1929, the League of Composers offered him $350 to premiere a chamber orchestra work. He suggested the newly published Symphony and indicated the strings could be performed solo despite the score markings. He confessed in his diary: "Better with multiple strings."

The wind section of Webern's Symphony is limited to clarinets and horns. Both instruments have wide ranges and strong pastoral connections. They are often paired together, and the horn is an archetypal symbol of wide Alpine space.

Webern saw variations as "the primeval form" which yields "the most comprehensive unity". He pointed to the second Netherland school's use of canons as the simplest method of variation because "everyone sings the same thing", and a great deal of material can be generated through simple methods like reversing or inverting the melody. Almost the entire Symphony is a series of canons, turning it into a kind of "essay in symmetry".

Webern used the same tone row for both movements. The material is even more cohesive because the second half of the row is derived from the first, a technique Webern relied on extensively. The first six notes (hexachord) are transposed up a diminished fifth and reversed to generate the last six notes. These symmetries are part of what make the Symphony one of the most unified works of its type. Since it is a mirrored row, there are only 24 permutations instead of the usual 48.