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, and The Beverly Hillbillies the lyrics of the theme song provide some necessary exposition for people unfamilar with the show.
In addition, some theme music uses orchestra scores or original music set mood for the show. One of the most notable of these is the Batman: The Animated Series theme song, which was drawn from the theme for the 1989 Batman film created by Danny Elfman. Others uses 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 created and 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 huge variety of styles, sometimes adapted from existing tunes, some composed specifically for the purpose. A few have been released commercially and become popular hits; examples would include the theme tune to Friends, "I'll Be There For You", was a hit for The Rembrandts and the theme from S.W.A.T. was a hit for Rhythm Heritage.
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.
Virtually every TV show has specific, melodic theme music, even if it's just a few notes (such as the snatch of eerie music that fades in and out in the title sequence for Lost). One famous exception is 60 Minutes, which features only the ticking hand of a Heuer stopwatch.
Some series use major hit songs that were popular before the shows' creation. A well known example is the CSI franchise, with each series having a well known song by The Who for their theme song. Another series, Murphy Brown, used a different classic Motown song each week as the show's opening theme.
Also notable is the Law & Order series, which started out with one theme song for Law and Order, and remixed it for its three spinoffs (Law & Order: Special Victims Unit, Law & Order: Criminal Intent, and Law & Order: Trial by Jury). The related reality show Crime and Punishment 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.