Incidental music is often "background" music, and adds atmosphere to the action. It may also include pieces which will provide the main interest for the audience, for example overtures, or music played during scene changes. It may also be required in plays which have musicians performing on-stage.
The use of incidental music dates back at least as far as Greek drama. A number of classical composers have written incidental music for various plays, with the more famous examples including Ludwig van Beethoven's music for Egmont, Felix Mendelssohn's music for A Midsummer Night's Dream and Edvard Grieg's music for Henrik Ibsen's Peer Gynt. Parts of all of these are often performed in concerts outside the context of the play.
More recently, incidental music has been written for computer games by thge likes of Michael Giacchino (the Medal of Honor series), Richard Jacques (Headhunter) and Jack Wall (Myst III). One of the best known incidental music composers for British television is Howard Goodall, who wrote music for The Gathering Storm, Blackadder and Red Dwarf as well as the film, Bean.