Delerium is a Canadian musical group that formed in 1987, originally as a side project of the influential industrial music act Front Line Assembly. Throughout the band’s history, their musical style has encompassed a broad range, including dark ethereal ambient trance, voiceless industrial soundscapes, and electronic pop music. They are best known for their worldwide hit "Silence", which features vocals from SarahMcLachlan. The song has been hailed as one of the greatest trance songs of all time, over a decade after its initial release.

Delerium is considered to be one of the pioneers in dark ambient and its variations, and is often confused for various other projects that have a similar sound to them, such as Deep Forest, Future Sound of London and Enigma. One of the two members of the band, Rhys Fulber; has gone onto a solo project, aptly named Conjure One; which has a lot of similarities to the older works of Delerium, but with more of a world ambient approach to their music. Delerium more recently, has become more and more electronic pop, while still paying tribute to a lot of their earlier works, continuing with more voiceless ambient/dark ambient soundscapes.

Delerium has traditionally been a two-person project, but the only constant member throughout its history has been Bill Leeb. Leeb was a guest musician and early supporter of industrial dance pioneers Skinny Puppy, but after he left in 1986 he went on to create his own project, Front Line Assembly with collaborator Michael Balch. Later, the two worked on the side project Delerium and released their first album, Faces, Forms & Illusions. After Balch left both Front Line Assembly and Delerium, Leeb worked with Rhys Fulber, and the two released several albums under the Delerium moniker; these years saw a gradual stylistic change from darker ambient to a more danceable sound.

After the release of Karma, Fulber left to pursue other interests, and Leeb teamed up with producer Chris Peterson to release Poem. 2003, however, saw the reunion of Leeb and Fulber for the release of Chimera, followed by Nuages du Monde in 2006, and for Music Box Opera in 2012.

In contrast to Leeb and collaborators' other projects, Delerium has included several guest vocalists since the release of Semantic Spaces. These have included mostly women, such as Kristy Thirsk, SarahMcLachlan, Leigh Nash (of SixpenceNoneTheRicher), Elsieanne Caplette (of Elsiane), Lisa Gerrard (sampled only), Jaël (of Swiss band Lunik), Camille Henderson, Nerina Pallot, Emily Haines (of Music/{{Metric}}), Jacqui Hunt (of Single Gun Theory), Isabel Bayrakdarian and Shelley Harland. Other than Leeb, only three males have contributed vocals to a Delerium album: Matthew Sweet ("Daylight", on Poem), Greg Froese ("Apparition", on Nuages du Monde), and Michael Logen ("Days Turn Into Nights", on the newest release Music Box Opera); in addition, the noted griot Baaba Maal was sampled ("Awakenings, on "Spiritual Archives").

The Mediæval Bæbes provided the vocal track for, and starred in the video of, Aria; the vocals are an adapted version of the vocals from "All Turns to Yesterday" on the Bæbes' Worldes Blysse album. They are also featured on two tracks from Delerium's 2006 album, Nuages du Monde.

Delerium is known for blending many different musical genres, while utilizing a heady amount of samples, being one of the most sample-heavy musical acts next to Skinny Puppy and Frontline Assembly, from which its two key members, Bill Leeb and Rhys Fulber are from. They touch on many themes and topics, most of them very mature and often dark and bittersweet, with many others making reference to sci-fi and fantasy themes, as best exhibited by their albums Spiritual Archives, Spheres, Spheres II, Semantic Spaces, Karma, Poem, Chimera and Nuages du Monde.

Although it may be argued that Front Line Assembly has the largest cult following of all Leeb and associates projects, Delerium is undoubtedly the most financially successful. In addition to these two mainstays, related projects of the Leeb, Fulber, Peterson, Balch family include Equinox, Intermix, Noise Unit, Pro-Tech, and Synæsthesia, among others. In addition, in 2007 Leeb and Fulber collaborated with Leigh Nash under the name Fauxliage, also Rhys Fulber maintains his solo project Conjure One since his temporary exit from Delerium.

Musical Acts like Balligomingo and Sleepthief have been greatly influenced by Delerium.

* AfterTheEnd: The music video for the track "Angelicus" as it is revealed bit by bit as the video goes along. The orphans seen in the video are what's left of Humankind.
* CherubicChoir: When they're not sampling them, Delerium does make use of them.
* IAmTheBand: For most of their career, Delerium was comprised of Bill Leeb and Rhys Fulber. After Rhys left in 2000 to create Conjure One, Bill remained the sole member of Delerium. in 2006, Rhys reunited with Bill to create Nuages du Monde, and then Music Box Opera in 2012.
* OneWomanWail: Delerium within recent years, has made heavy usage of predominant female vocals, with the occasional male vocalist here and there. Despite misconceptions, Delerium does not saturate their vocal tracks with OneWomanWail.
** The track Forgotten Worlds samples Lisa Gerrard's Persian Love Song, and the track Angelicus has Canadian Opera Singer Isabel Bayrakdarian providing the track's trademark haunting vocals.
** Two of their tracks make use of the multiple female vocalists of the Medieveal Babes, breaking away from their usage of solo or duo female vocals.
** The compilation album, Voices; features many of the female vocalists who have worked with Delerium, providing their vocals once more for acoustic and orchestral renditions of the tracks they worked on.
* OminousLatinChanting: Very prominent in their earlier works, but not so much lately.
* RuleOfSymbolism and ShoutOut: The works of Delerium are infused with this, as well as the many side projects of Bill Leeb and Rhys Fulber.
** Many of the track names on any given Delerium album and any album of the related side-projects by Bill Leeb and/or Rhys Fulber, are symbolically tied into the track itself, or a thematic shoutout related to the track.
** For their first album of Delerium, Faces, Forms and Illusions, The front cover of the album shows a photo of Thich Quang Duc, a Vietnamese Buddhist monk setting himself alight and burning to death in Saigon, 1963. The people in the background are other monks and Vietnamese citizens who had gathered to watch the burning monk's suicide. A photo of exactly the same event, although taken from a different angle, was later used on the debut album for Rage Against The Machine. The title, Faces, Forms and Illusions is a reference to the illusion of reality and the impermanence of life and death.
** Spheres and Spheres II both make reference to outer space, other worlds, space related phenomena and various sci-fi themes, and goes as far as to sample NASA Space Mission Recordings and clips from 2001: A Space Odyssey.
** Semantic Spaces, Karma, Poem, Chimera, Nuages du Monde and Music Box Opera are all very heavily infused with Fantasy elements and imagery, as well as Urban Fantasy elements and imagery. Many of the tracks and track names deal with themes of loneliness, alienation, memories and the nature of identity and memory, heartbreak, loss and sorrow, lust, longing, one-night stands, and various others.
* Sampling: A very common mainstay in the works of Delerium. They are 3rd most sample-heavy electronica project on the planet. They sample everything from obscure Middle-Eastern and European folk songs to old-school sci-fi movies.
* SmallNameBigEgo: Averted big time with Bill Leeb and Rhys Fulber. Despite their popularity and their success, both men have remained very humble and private. Rhys is one of the most sought after producers in the music industry and has gone onto producing and working on countless albums. Bill himself has contributed to various other projects, and provided the soundtracks for the video games, Quake III Arena(as a part of Sonic Mayhem) and AirMech.
* UrbanFantasy: The music videos for the tracks "Dust in Gravity" and "Monarch".