The original non-diegetic music, or musical score, to Lost is composed by Michael Giacchino and performed by members of the Hollywood Studio Symphony. The instrumentation consists primarily of a string section, four trombones, piano, harp, and assorted percussion instruments. Occasionally additional instruments are employed such as synthesizer, guitar, and ukulele. Tim Simonec is the conductor of the orchestra, and Dan Wallin is the recording and mixing engineer.
The score makes extensive use of leitmotifs, which are musical themes that appear for specific characters or circumstances. For instance, Kate has a theme which is heard at various times when the story focuses on her character (titled "Kate's Motel" on the Season 1 soundtrack; first introduced in "Born to Run"). As well, most of the main characters have one or more themes. There is also a theme, for example, when people are exploring the Island, often for the first time, as heard on the first mission to the cockpit in "Pilot, Part 1", the trek to the Black Rock in "Exodus, Part 1" and on Juliet's arrival to the Island in "One of Us". The leitmotifs are usually short, often used as repeating figures, and sometimes undergo several variations throughout the series. Occasionally, different themes are integrated together when different characters or circumstances are present at the same time.
An example of one the leitmotifs is "Hollywood and Vines" (Soundtracks). This theme has appeared several times ("Pilot, Part 2") ("Exodus, Part 2") ("Abandoned") ("Through the Looking Glass, Part 1") ("Cabin Fever") ("Follow the Leader"), and is used when characters are trekking across the Island (often to higher elevation). It opens with a recognizable and prominent bass figure, which is phrased in a syncopated rhythmic pattern, giving it a somewhat staggered feel. Its minor key, low register, and heavy reliance on the tonic and dominant notes of the scale add to it a tone of fortitude and gravity.
"Hollywood and Vines":
Jack's leitmotif is discussed by composer Michael Giacchino in the DVD extra "Lost in a Day" on the Lost: The Complete Third Season (DVD) (at the 5:41 pm Los Angeles segment). Actor Matthew Fox learned to play the song on piano so that he could be filmed playing his own leitmotif.
Recurring sonic textures
The use of 'stylistic themes', or recurring sonic textures, plays a large role in giving the musical score a unique and identifiable sound. Often, the instruments in the ensemble are played in uncharacteristic ways, and they are utilized for their particular acoustic qualities. This approach generally has the effect of establishing mood and atmosphere. Some well-known examples of these recurring sonic textures include:
- The familiar trombone "fall-offs" that often occur right before a commercial break.
- Themes played as very slow, quiet passages on piano.
- Frantic action music, typically relying on exotic percussion (see below) and repeating figures in the strings and brass.
- String instruments played near the bridge of the instrument (known as "sul ponticello"), producing a tense, eerie, or otherworldly sound.
- Slowly repeating harp notes, particularly in the low register, often used for quiet suspense.
- The use of unconventional percussion instruments as sound effects:
- A metallic Angklung, which gives a high pitched, "jangly" sound, is used for moments of uneasiness and fear.
- A Flapamba (a marimba-like instrument) is often used, usually in a rapid sequence of low, muffled notes.
- Percussionist Emil Richards drags a superball across a gong to produce an eerie howling sound. (Official Lost Podcast/January 09, 2006 (Orchestra Special)) Examples of this effect in use include, 1) when Cooper is revealed to Locke at the end of "The Man from Tallahassee", and 2) coinciding with the orchestral punctuations in the final shot of "Exposé", showing the mound of sand over Nikki and Paulo.
- The insides of a piano were taken out, and are used to strike the strings in various uncustomary ways. (Official Lost Podcast/January 09, 2006 (Orchestra Special))
- Scraps of metal from the crashed airplane were shipped back to Los Angeles by producer Bryan Burk so that they could be used as percussion instruments. (Welcome to Oahu) This is an example of "found percussion".
- The occasional use of waterphone.
- In quieter, emotional scenes, a certain chord is often used. It consists of the tonic, the note a half-step below that, and the note a major third below that.
- After Shannon sings "La Mer" to Sayid in "Whatever the Case May Be", the piano continues playing the song through the end of the episode.
- After "These Arms of Mine" fades out near the end of "S.O.S.", the strings play a short phrase from the song during the shot of Rose and Bernard sitting by the fire.
- After "Shambala" comes on in the DHARMA van in "Tricia Tanaka Is Dead", the song is integrated into the score for use in the following montage sequence.
- After David Shephard begins his recital of Chopin's Fantasie Impromptu in C# minor in Lighthouse, the song is integrated into the score.
There have been two instances where non-diegetic music was used that was a commercial song, not composed by Giacchino.
- "I Shall Not Walk Alone" is heard as Sayid leaves the camp in "Confidence Man".
- "Slowly" is heard at the beginning of flashback in "I Do".
Process and approach
From the beginning of the series, Michael Giacchino's approach to scoring Lost has always been to try to avoid jungle and action film clichés, like flute and woodwind instruments, instead opting for an unusual and disorienting tonal palette. Also, as a creative strategy, Giacchino doesn't read the scripts ahead of time, instead choosing to watch the episode until reaching a point that needs music, and then writing that cue.  (Official Lost Podcast/April 10, 2007)
The process of composing and orchestrating an episode's score takes two days, and the orchestra and recording team have three hours to record all of the music at the scoring stage. The musicians in the orchestra don't rehearse beforehand, rather they come in to the session and sight-read their parts.  (Official Lost Podcast/April 10, 2007) (Official Lost Podcast/January 09, 2006 (Orchestra Special))
- The end credits theme music on Lost is a combination of two separate score elements from "Pilot, Part 1". The rhythmic bed is the 14 bars of percussion heard when Jack is telling Hurley to watch Claire's contractions shortly after the crash (5:17–5:45). Layered on top of that with mallet percussion is the melodic leitmotif first introduced when Jack is running out from the jungle to the crash site. (The Eyeland) Strings slowly rising in pitch and a trombone swell at the end are also added.
- In the show, Giacchino has referenced his own work outside of Lost. The melody played by the low strings when Locke is exploring the submarine in "The Man from Tallahassee" is from "The U-Boat", a piece from the video game Medal of Honor, also composed by Giacchino.  He later used this theme again for scenes involving the submarine in "Follow the Leader" and "The Incident, Part 1".
Original diegetic music
Giacchino on occasion has composed diegetic music, heard by the characters in the show:
- In "Born to Run", when Charlie shows Claire a new song that he's writing for Drive Shaft ("Monster Eats the Pilot"), the riff that he plays on his guitar is based on the melody to "Credit Where Credit is Due" (Soundtracks), a prominent theme first heard in "Pilot, Part 1".
- The theme songs for the Swan and Pearl Orientation videos contain the melody (with an added note at the end) from "The Eyeland" (Soundtracks), which appears in the end credits of the show and also in the score.
- In "Not in Portland", the breakbeat music when Karl is being brainwashed in Room 23 was composed by Giacchino specifically for the scene. 
- The song that Jack plays on piano in "The Man from Tallahassee" is his leitmotif, which was composed by Giacchino for Season 3. (Official Lost Podcast/March 26, 2007)
Michael Giacchino has won several awards for his work on the musical score.
- 2005 Emmy Award - "Outstanding Music Composition for a Series (Dramatic Underscore)"
- 2005 ASCAP Film and Television Music Awards - "Top TV Series", with J.J. Abrams
- 2005 BMI Film & TV Awards - "BMI TV Music Award"
- 2006 ASCAP Film and Television Music Awards - "Top TV Series", with J.J. Abrams
- Michael Giacchino
- Soundtracks – All six seasons have soundtrack albums released, featuring music from the show.
- Musical themes
- The Lost Symphony – A symphonic work for orchestra featuring music from the show.
- Lost Live: The Final Celebration
- Songs featured in Lost
- January 9, 2006 Official Lost Podcast – A special orchestra edition in which Michael Giacchino takes a tour of the scoring stage, and introduces musicians and instruments that make up the sound of Lost
- April 10, 2007 Official Lost Podcast – A video podcast in which Giacchino is interviewed and takes a tour of the scoring stage
- NJ Star-Ledger – June 22, 2008 interview with Michael Giacchino
- New Yorker: Out Loud podcast – Music critic Alex Ross talks about Giacchino and the Lost musical score