A symphony is an extended musical composition in Western classical music, scored almost always for orchestra. "Symphony" does not necessarily imply a specific form, though most are composed according to the sonata principle. Many symphonies are tonal works in four movements with the first in sonata form, which is often described by music theorists as the structure of a "classical" symphony, although many symphonies by the acknowledged classical masters of the form, Joseph Haydn, Wolfgang Amadeus Mozart, and Ludwig van Beethoven do not conform to this model.

Early symphonies, in common with both overtures and ripieno concertos, have three movements, in the tempi quick-slow-quick. However, unlike the ripieno concerto, which uses the usual ritornello form of the concerto, at least the first movement of these symphonies is in binary form. They are distinguishable from Italian overtures in that they were written to stand on their own in concert performances, rather than to introduce a stage work, although a piece originally written as an overture was sometimes later used as a symphony, and vice versa. Symphonies at this time, whether for concert, opera, or church use, were not considered the major works on a program: often, as with concerti, they were divided up between other works, or drawn from suites or overtures. Vocal music was dominant, and symphonies provided preludes, interludes, and postludes. The "Italian" style of symphony, often used as overture and entr'acte in opera houses, became a standard three movement form: a fast movement, a slow movement, and then another fast movement. Haydn and Mozart, whose early symphonies were in this layout, eventually replaced it with a four-movement layout, through the addition of an additional middle movement (Prout 1895, 249), which became dominant in the latter part of the 18th century and most of the 19th century. This symphonic form was influenced by Germanic practice, and would come to be associated with the "classical style" of Haydn and Mozart. "Normative macro-symphonic form may be defined as the four-movement form, in general, employed in the later symphonies of Haydn and Mozart, and in those of Beethoven" (Jackson 1999, 26)

With the rise of established professional orchestras, the symphony assumed a more prominent place in concert life between approximately 1790 and 1820. Beethoven dramatically expanded the symphony. His Symphony No. 3 (the Eroica), has a scale and emotional range that sets it apart from earlier works. His Symphony No. 5 is arguably the most famous symphony ever written. His Symphony No. 9 takes the unprecedented step (for a symphony) of including parts for vocal soloists and choir in the last movement, making it a choral symphony (however, a minor composer, Daniel Steibelt had written a piano concerto with a choral finale four years earlier, in 1820). Hector Berlioz, who coined the term "choral symphony", built on this concept in his "dramatic symphony" Roméo et Juliette while explaining his intent in the five-paragraph introduction in that work's score (Berlioz 1857, 1). In Beethoven's Pastoral Symphony, a program work, the composer inserted a "storm" section before the final movement; Berlioz's Symphonie fantastique, a work famous for its exceptional orchestration is also a programme work and has both a march and a waltz and five movements instead of the customary four.

At the beginning of the 20th century, Gustav Mahler wrote long, large-scale symphonies. His Eighth Symphony, for example, was composed in 1906 and is nicknamed the "Symphony of a Thousand" because of the forces required to perform it. The 20th century also saw further diversification in the style and content of works that composers labeled symphonies (Anon. 2008). 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, whereas Alan Hovhaness's Symphony No. 9, Saint VartanвАФoriginally op. 80, changed to op. 180вАФcomposed in 1949вАУ50, is in twenty-four.