Theme music is a piece that is often written specifically for a radio program, television program, video game or movie, and usually played during the intro, during title sequence and/or ending credits. If it is accompanied by lyrics, most often associated with the show, it is a theme song.
The phrase theme song or signature tune may also be used to refer to a song that has become especially associated with a particular performer or dignitary; often used as they make an entrance.
The purpose of a theme song is often similar to that of a leitmotif.
The purpose of the music is to establish a mood for the show and to provide an audible cue that a particular show is beginning, which was especially useful in the early days of radio (See also interval signal). In some cases, including The Brady Bunch, Gilligan's Island, The Fresh Prince of Bel-Air, Land of the Lost, The Nanny and The Beverly Hillbillies the lyrics of the theme song provide some necessary exposition for people unfamiliar with the show.
In addition, some theme music uses orchestra scores or original music set mood for the show, such as the Batman: The Animated Series theme song, which was drawn from the theme for the 1989 Batman film created by Danny Elfman and sets the mood for the cartoon. Another example is Ron Grainer's theme music for Doctor Who. Other shows use remixes or covers of older songs, such as the theme song of Spider-Man: The Animated Series (1994-1998), which featured a reworked cover of the theme song from the classic Spider-Man cartoon from the 1960s. The song was performed by Aerosmith lead guitarist Joe Perry.
Theme music has been a feature of the majority of television programs since the medium's inception, as it was for the ancestral radio shows that provided their inspiration. Programs have used theme music in a large variety of styles, sometimes adapted from existing tunes, and with some composed specifically for the purpose. A few have been released commercially and become popular hits; examples include the title theme from Rawhide, performed and recorded by popular singer Frankie Laine; the theme to Happy Days (from 1974-84), performed by Pratt & McClain (in Top 5, 1976); the theme to Laverne & Shirley, performed by Cyndi Grecco (#25, 1976); the theme to Friends, "I'll Be There For You", which was a hit for The Rembrandts; the theme from S.W.A.T., which was a hit for Rhythm Heritage, the theme song from Pokemon, which is a long time hit for Jason Paige, and the theme song from Drake & Josh, which was a hit for Drake Bell. Jan Hammer had a major hit with the theme from Miami Vice in the 1980s. "Theme From Dr. Kildare (Three Stars Will Shine Tonight)", recorded by Richard Chamberlain, the star of the television series, was in 1962 a top 10 hit in the U.S. and a top 20 hit in the UK.
Other themes, like the music for The Young and the Restless, Days of our Lives, and Coronation Street have become iconic mostly due to the shows' respective longevities. Unlike others, these serials have not strayed from the original theme mix much, if at all, allowing them to be known by multiple generations of television viewers.
In the United Kingdom, iconic sports shows have such strong associations with their theme music that the sports themselves are synonymous with the theme tunes, such as football (Match of the Day theme), cricket (Booker T. & the M.G.'s, "Soul Limbo"), motor racing (Roger Barsotti's Motor Sport and the bassline from Fleetwood Mac's The Chain), tennis (Keith Mansfield's Light and Tuneful), snooker (Drag Racer by the Doug Wood Band), and skiing (Pop Goes Bach, the theme to Ski Sunday). Themes in the United States that have become associated with a sport include Johnny Pearson's "Heavy Action" (used for many years as an intro to Monday Night Football), "Roundball Rock" (composed by John Tesh as the theme for the NBA on NBC during the 1990s and early 2000s), "Bugler's Dream" (used in ABC and NBC's coverage of the Olympics) and the theme to ESPN's nightly sports highlight show, SportsCenter. A notable theme that was once associated with a sport, but because of its popularity, spread network-wide was the NFL on Fox theme, which is now used for MLB on Fox and NASCAR on Fox broadcasts, and is regarded as the network's single theme by October 2010.
Most television shows have specific, melodic theme music, even if just a few notes (such as the clip of music that fades in and out in the title sequence for Lost, or the pulsing sound of helicopter blades in the theme music for Airwolf). One exception is 60 Minutes, which features only the ticking hand of a Heuer stopwatch. Another recent exception is Body of Proof which has no theme song, and barely even has a title sequence.
Also notable is the Law & Order series, which started out with one theme song for Law & Order, and remixed it for its four spinoffs (Law & Order: Special Victims Unit, Law & Order: Criminal Intent, Law & Order: Trial by Jury and Law & Order: LA). The related shows Crime and Punishment, New York Undercover. Arrest & Trial and Stars Earn Stripes also aired with a remix of the theme.
Radio programs with notable theme music include Just a Minute, which uses a high-speed rendition of the Minute Waltz by Frédéric Chopin; The Archers, which has Barwick Green; Desert Island Discs which has By the Sleepy Lagoon, and The Rush Limbaugh Show, which uses the instrumental from "My City Was Gone."
In talk radio, a different theme song is often used to introduce each segment, and the music (usually popular music of some sort) will often relate to the topic being discussed. John Batchelor is noted for his use of highly dramatic orchestral scores leading in and out of each segment of his weekly show.
Video games 
Many video games feature a theme song that is distinctive to the series. A popular one to date is the Prelude Theme from the Final Fantasy Series, which is played on most, if not all, of the title screens of the original games, most notably Final Fantasy I to Final Fantasy IV. The newer ones also feature the theme, albeit usually modernized, and played during the ending credits.
See also 
- " Another notable one is the theme music that has played in either the title screen or opening scene of the main series Pokemon games, which slightly changes with each generation. The History of Final Fantasy", GameSpot.com.
- "The Evolution of Final Fantasy: From I to XII, we catch you up on the facts.", Au.PS2.IGN.com (Australia, January 19, 2007).