Leitmotifs

A leitmotif (/ˌlaɪtmoʊˈtiːf/) is a musical "phrase" or melody, re-used in multiple songs to associate a tune with a person, event, or place.

The extensive use of leitmotifs in Undertale ' s soundtrack and other sound files helps create a nostalgic mood for the game and often evokes emotional responses in players. Toby Fox has stated that the majority of Undertale ' s soundtrack was composed using reiterations of free SoundFonts and synths.

Attempts have been made at documenting all of the leitmotifs used in the game; players frequently find new connections, and it may be impossible to list them all conclusively.

Timestamps found below are approximate to one second and are sourced from this playlist, which reflects the timing of the official soundtrack.

Once Upon a Time
This is one of the most frequently recurring melodies in the game. The original song can be divided into three sections, which together play in 17 songs.

"A" section

 * Once Upon a Time 0:00–0:28
 * Start Menu and Menu (Full)
 * The background tracks for the Start Menu build upon one another, with five intermediate tracks between Start Menu and Menu (Full).
 * The music played in the menu at the end of a True Pacifist playthrough is a slowed rendition of Start Menu.
 * Home and Home (Music Box) 0:36–1:11
 * Hotel 0:20–0:41
 * Can You Really Call This A Hotel, I Didn't Receive A Mint On My Pillow Or Anything 0:14–0:30
 * Undertale 0:37–2:49, 3:27–4:43, 5:20–5:58
 * The Choice
 * Fallen Down (Reprise) 1:18–1:44
 * Hopes and Dreams 0:00–0:44
 * SAVE the World 0:00–0:20, 1:46–1:53
 * Reunited 0:43–1:43
 * Bring It In, Guys! 3:27–3:35, 3:47–4:05
 * Last Goodbye 0:23–0:27, 0:45–0:48, 0:56–1:02, 1:40–1:42, 1:52–2:02
 * Good Night

"B" section

 * Once Upon a Time 0:28–0:58
 * Home and Home (Music Box) 1:11–1:45
 * Hotel 0:41–1:22
 * Can You Really Call This A Hotel... 0:30–0:57
 * Undertale 2:49–3:27
 * Fallen Down (Reprise) 1:44–2:10
 * Reunited 1:43–2:42
 * Last Goodbye 0:02–0:21, 1:08–1:29

"C" section

 * Once Upon a Time 0:58–1:29
 * Fallen Down (Reprise) 2:10–2:30
 * Hopes and Dreams 0:44–1:29, 1:52–2:15
 * SAVE the World 0:42–1:46
 * Final Power
 * Reunited 2:56–3:56
 * Last Goodbye 0:45–1:08

Flowey
This leitmotif occurs in at least six songs, as well as in at least one other sound file within the game.
 * Your Best Friend
 * Your Best Nightmare 0:54–1:26, 2:00–2:27, 3:00–3:27
 * Finale 0:00–0:40, 0:57–1:52
 * Hopes and Dreams 1:29–1:52
 * SAVE the World 0:04–0:07 repeating
 * Last Goodbye 0:29–0:32 repeating
 * mus_toomuch (this plays after exhausting the random kill counter of an area)

Fallen Down

 * Fallen Down
 * Fallen Down (Reprise) 0:00–1:18
 * Bring It In, Guys! 3:35–3:45

The music in-game {which track/s?} is played 25 cents above C# major in A440 equal-tempered tuning. The sheet music is written in D major.

Heartache

 * Heartache 0:00–0:50
 * ASGORE 0:34–1:49, syncopated to 4/4

Ruins
Ruins is another common melody, appearing in at least 11 official songs and an unused sound file.
 * Ruins
 * Waterfall 0:00-0:26 (Ruins ostinato transformed into the Area Bells ostinato), 0:27–1:49
 * Quiet Water 0:05–0:32
 * NGAHHH!! 0:34–1:22
 * Spear of Justice 0:36–1:52
 * An Ending
 * Don't Give Up
 * Respite 0:35–1:54
 * Bring It In, Guys! 1:48–2:08
 * But the Earth Refused to Die
 * Battle Against a True Hero 0:44–1:36
 * mus_ruinspiano

Anticipation
Two to three seconds of the Anticipation leitmotif is somewhat audible within the Ghost Fight leitmotif.
 * Anticipation
 * This song plays at extremely reduced speeds when battling Monster Kid, during the final encounters with Flowey in New Home, and at the end of a "Soulless" Pacifist playthrough.
 * Enemy Approaching
 * Heartache 0:54–0:57, 0:59–1:01
 * Dogsong, this version has the variation of the melody.
 * A slowed version of the song plays after meeting Endogeny's sparing conditions.
 * Temmie Village 0:11–0:16 repeating, also has the variation
 * Tem Shop, also has the variation
 * Stronger Monsters 0:00–0:32, 0:38–1:03
 * Bring It In, Guys! 0:00–0:40

Ghost Fight
Five songs.
 * Ghost Fight
 * Dogbass (quotes the bassline from 0:08-0:16)
 * Dummy!
 * Pathetic House
 * Spider Dance

Sans
Found in four songs, one of which is never played in the game. One version can be affected by a non-Neutral Route.
 * sans.
 * It's Raining Somewhere Else 0:19–1:18, 2:00–2:39
 * This song is most similar to the B section (0:10-0:17) of Song That Might Play When You Fight Sans.
 * This song is played slightly slower in Sans's Room.
 * Song That Might Play When You Fight Sans 0:01–0:33 (melody), 0:01–0:58 (bassline)
 * Bring It In, Guys! 0:43–0:46 (melody), 0:40–0:46 (bassline)

Papyrus

 * Nyeh Heh Heh!
 * Bonetrousle
 * This song is played at an extremely reduced speed when encountering Papyrus for the last time in the Genocide Route.
 * Song That Might Play When You Fight Sans 0:42–0:58
 * Bring It In, Guys! 0:35–0:54

Snowdin

 * Snowy 0:00–1:02
 * This song plays when fighting Snowdrake's Mother. It is heavily distorted, changing pitch and speed randomly every few seconds.
 * Because this song does not contain the second half of the melody found in the other songs, it is not always considered the origin of the leitmotif.
 * Snowdin Town 0:19–1:16
 * Shop
 * Dating Start! 0:00–1:13, somewhat syncopated.
 * Dating Fight!
 * Confession
 * Hopes and Dreams 2:15–3:01
 * Reunited 3:56–4:25
 * Bring It In, Guys! 0:54–1:29

Undyne

 * Dating Tense!
 * Undyne
 * Run!
 * NGAHHH!! 0:00–0:34, syncopated
 * Spear of Justice 0:00–0:36
 * Bring It In, Guys! 1:30–1:48
 * ASGORE 2:29-2:35

Asriel

 * Memory
 * Undertale 0:00–2:47, 3:27–6:21
 * At times, the melody is barely perceptible amongst overlapping music. It is heard on a guitar for most of the song, becoming hard to distinguish after 3:27. The tune switches to a more audible sample after 4:42, to a quiet background piano at the key change around 5:20, and back to the second SoundFont at 5:57.
 * The Choice (imperceptible), as it is a part of Undertale which contains the barely-audible Memory
 * Finale 0:40–0:57
 * His Theme
 * mus_star (the first four notes in each eight-note segment){clarify}

"A" section

 * Alphys 0:08–1:00
 * Bird That Carries You Over A Disproportionately Small Gap 0:06–0:13
 * Here We Are, syncopated to 5/4

"B" section

 * Alphys 1:00–1:25
 * She's Playing Piano
 * Here We Are 1:28 - 1:50

Hotland

 * Another Medium 0:51–1:21, 1:50–2:14
 * CORE 0:34–1:01, 1:42–2:10
 * Bring It In, Guys! 2:44–3:04

Showtime
Found in four songs and at least one other sound file.
 * It's Showtime!
 * mus_tv (i.e., Papyrus's TV)
 * Live Report
 * Death Report
 * Death by Glamour 1:30–1:56

Metal Crusher

 * Metal Crusher 0:19-0:36
 * Last Episode!
 * Death by Glamour 1:04–1:30

Hotel Intro

 * Hotel 0:00–0:20
 * Can You Really Call This A Hotel... 0:00–0:15
 * CORE Approach

Musical
Hotel-related songs are largely part of the Once Upon a Time leitmotifs.
 * Oh! One True Love
 * Oh! Dungeon
 * For The Fans

Area Bells
The theme originates from the piano ostinato from Ruins.
 * Waterfall 0:00–0:27
 * Another Medium 0:00–0:51, 1:50–2:22
 * CORE 0:06–1:01, 1:29–2:10

Bergentrückung
Asgore's main theme is uncommon, appearing in only 3 tracks.
 * Bergentrückung
 * ASGORE 0:00–0:35, 1:25–1:32
 * Bring It In, Guys! 3:28–3:34

Determination

 * Determination
 * This song is sped up, looped, and played over itself, as Flowey laughs at the protagonist for dying to him.
 * Alphys 0:31–0:59 (debatable)
 * ASGORE 1:50–2:28
 * mus_snoresymphony
 * mus_kingdescription

Your Best Nightmare Intro

 * Premonition
 * You Idiot
 * Your Best Nightmare 0:00–0:15
 * Burn in Despair! (background chords)

Gaster's Theme
Gaster's Theme is a song that only plays in the Sound Test room. The Sound Test room, in turn, is only found half the time between rooms 46 (room_tundra_3, Snowdin – Box Road) and 47 (room_tundra_3A), and only when fun = 65.

Four-tone accompaniments are fairly common; any of the following could be coincidental, making all of them speculative.
 * Nyeh Heh Heh! 0:00–0:01 repeating (debatable)
 * Bonetrousle 0:00–0:01 repeating (debatable)
 * Dating Start! 0:02–0:03 repeating
 * Song That Might Play When You Fight Sans 0:10 repeating

Minor Similarities

 * The Joystick Config songs – Uwa!! So Temperate♫, ...Holiday♫, and ...HEATS!!♫ – are all variants of each other.
 * Spookwave to Spooktune
 * Ghouliday to Jingle Bells
 * Sample used in Barrier is used in Amalgam
 * Sample used in Ooo is used in Waterfall {add times}
 * Battle Against a True Hero 0:00–1:36 to Power of "NEO"
 * mus_smile is Muffet's laugh, sped up, reversed, and looped.
 * Bonetrousle to the pre-battle Mad Dummy music (thus, possibly also Gaster's Theme) {be more specific}

Megalovania
This song is part of a unique leitmotif. The melody of MEGALOVANIA is not heavily recycled inside the game; instead, this leitmotif crosses between games, appearing most notably in Toby Fox's own Earthbound Halloween Hack, the seven-year webcomic Homestuck, and in the Mother fangame Cognitive Dissonance. Future remixes are slated for use in future Undertale-based fangames. The song's use as a leitmotif is associated with an endgame event or boss battle, bringing a familiar sense of finality and the absolute to the games in which it is used.

Careful analysis of Spooktune (and therefore, Spookwave) shows that the songs have very similar melodies with parts of MEGALOVANIA {add times}, though this may be coincidental.