Leeb was an early member of industrial music pioneers Skinny Puppy, but after he left in 1986 he went on to create his own project, Front Line Assembly, with collaborator Michael Balch.

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 (of Rose Chronicles), Sarah McLachlan, Leigh Nash (of Sixpence None the Richer), Elsieanne Caplette (of Elsiane), Lisa Gerrard (sampled only), Jaël (of Swiss band Lunik), Camille Henderson, Nerina Pallot, Emily Haines (of 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 Music Box Opera); in addition, the noted griot Baaba Maal was sampled ("Awakenings, on "Spiritual Archives").

In addition to these two mainstays, related collaborations between Leeb and Fulber, Peterson, and/or Balch include Equinox, Intermix, Noise Unit, Pro-Tech, and Synæsthesia, among others.