Incidental music is that music which cannot be heard by the characters in the narrative. It is used for the benefit of the audience, in order to give emotional context for a scene.
Doctor Who's relationship with incidental music was spotty during the 1960s. Many stories did not in fact have original incidental music, instead employing accompaniments of special sounds and/or stock music. It was really only with the rise of Barry Letts as producer that serials began to consistently utilise genuine, specially composed incidental music scores.
During the 1970s, incidental music was — with a few important exceptions — typically composed by Dudley Simpson, and performed using acoustic instruments or with the limited addition of a synthesiser. Though the budget did not run to full orchestras, Simpson was typically able to have at least a few live instrumentalists, such that the major sections of the orchestra were represented.
Upon installation, producer John Nathan-Turner, however, politely fired Simpson. From The Leisure Hive onwards the incidental music was mainly performed on synthesisers by the BBC Radiophonic Workshop, with some exceptions allowing for additional instruments such as electric (Paddy Kingsland's work) or acoustic (The Two Doctors) guitar. Later series employed freelance composers who utilised commercially available synthesisers, a practice which continued until Survival.
John Debney's score for the 1996 tele-movie made exclusive use of a virtual orchestra. When Doctor Who returned in 2005, Murray Gold was initially only able to provide the same, supplemented by an orchestral session for the Doctor Who theme and some choral performances. A full orchestral range of sound was unavailable until the budget was increased following the success of Series 1. From The Christmas Invasion forward, Doctor Who enjoyed genuine orchestral incidental music, to the extent that the show's regular orchestrator, Ben Foster, became almost as recognisable as composer Murray Gold.
Modern Doctor Who has also occasionally obtained the rights to use pre-existing popular music as part of the score. Songs like "Englishman in New York", "Sunshine", and "Chances" have been used as scene-setters.
Sometimes, rough prints of Doctor Who episodes have featured unofficial music tracks in the interim before being replaced with new orchestrated soundtracks, such as when these unfinished cuts of episodes are privately screened. In the final version of an episode, the style or mood of the music used in a scene may be changed from that of the rough print, or music may be removed from a scene entirely.