Aliens was one of Horner's first major Hollywood scores, and the project was a traumatic one for the young composer. When he arrived in London to begin writing, he expected the film to be "locked", giving him six weeks to assemble the score, a time-frame he considered sufficient. However, he actually discovered that filming and editing were still taking place, and he was unable to view the film or begin his work. Horner visited the sets and editing rooms for three weeks and found that editor Ray Lovejoy was barely keeping up with the workload. Horner believed director James Cameron was more concerned with sound effects, citing the fact that Cameron spent two days with the sound engineer creating the firing sounds for the Pulse Rifles. Horner also complained that he was given an outdated recording studio; the score was recorded with the London Symphony Orchestra at Abbey Road Studios, a thirty-year-old facility that was barely able to patch in synthesizers. As a result, Horner was forced to abandon many of the more advanced electronic elements he had intended to employ.
Six weeks from the film's theatrical release, no dubbing had taken place and editing was ongoing, and as a result the score had still not been written. Producer Gale Anne Hurd had only limited music production experience, and she and Cameron denied Horner's request to push the film back four weeks so he could finish the score. At one point, Hurd even threatened to fire Horner in response to his continued complaints, to which Horner simply replied that if she could find someone who was physically capable of meeting their deadline, she was welcome to replace him. Ultimately, Horner remained and was forced to rush through composing the music in just four days. The final cue composed was that which accompanied the finale of the film, when the Queen is sucked out into space through the Sulaco's airlock. However, to Horner's horror, Cameron then completely reworked the scene, forcing Horner to spend the next 36 hours desperately rewriting the music. Ironically, the cue is now considered the most famous from the entire film; according to Hurd, "It became the signature cue not only in [Aliens], but it's been in trailer, after trailer, after trailer." The music was even used in trailers for Alien3.
Following release, Horner felt that his score was only around 80% of what he hoped to achieve, whereas with more time he could have increased that to 100%. Despite this, Horner received an Academy Award nomination (his first) for Best Original Score. However, owing to the tensions between himself and Cameron, he decided at the time never to work with the director again, believing the his film schedules were too short and stressful. Horner would keep this promise until 1997, when Cameron, impressed with Horner's soundtrack for Braveheart, approached him to compose the score for Titanic. The two reconciled, and Horner would go on to score music for Cameron's next film, Avatar.
The original release featured just under 40 minutes of music from the movie, although the cues on the album were not in the order they appeared in the film. In 2001, Varèse Sarabande re-released an expanded Deluxe Edition, featuring an expanded and chronological score with bonus tracks.
- "Main Title" — 5:10
- "Going After Newt" — 3:08
- "Sub-Level 3" — 6:11
- "Ripley's Rescue" — 3:13
- "Atmosphere Station" — 3:05
- "Futile Escape" — 8:13
- "Dark Discovery" — 2:00
- "Bishop's Countdown" — 2:47
- "Resolution and Hyperspace" — 6:10
2001 Deluxe Edition
- "Main Title" — 5:13
- "Bad Dreams" — 1:22
- "Dark Discovery/Newt's Horror" — 2:07
- "LV-426" — 2:03
- "Combat Drop" — 3:29
- "The Complex" — 1:34
- "Atmosphere Station" — 3:11
- "Med Lab" — 2:04
- "Newt" — 1:14
- "Sub-Level 3" — 6:36
- "Ripley's Rescue" — 3:19
- "Facehuggers" — 4:24
- "Futile Escape" — 8:29
- "Newt Is Taken" — 2:04
- "Going After Newt" — 3:18
- "The Queen" — 1:45
- "Bishop's Countdown" — 2:50
- "Queen to Bishop" — 2:31
- "Resolution and Hyperspace" — 6:27
- "Bad Dreams (Alternate Version)" — 1:23
- "Ripley's Rescue (Percussion Only)" — 3:20
- "LV-426 (Film Version)" — 1:13
- "Combat Drop (Percussion Only)" — 3:24
- "Hyperspace (Alternate Version)" — 2:08
- The score for Aliens includes elements from Jerry Goldsmith's soundtrack for Alien at various points (although not all of these moments are included on the soundtrack album). In fact, one particular piece of Goldsmith music was recycled for Aliens totally unchanged; it plays when Ripley is cornered by the Queen at the base of the elevator shaft. Interestingly, the scenes in Alien and Aliens that utilize this piece of music are thematically very similar — in both instances, the track plays when Ripley is cornered by a Xenomorph whilst trying to flee a countdown to an impending explosion. Alien Resurrection and Prometheus would later also incorporate elements of Goldsmith's Alien score.
- The first part of the track "Resolution and Hyperspace" was not used in Aliens. However, the unsued section did appear in the finale of the 1988 action movie Die Hard (directed by John McTiernan), used when terrorist Karl emerges from the Nakatomi building after his apparent death and attempts to kill John McClane. The track was originally only placed in Die Hard as temporary source music until the film's composer, Michael Kamen, could write his own cue for the scene. However, director McTiernan liked it so much the track was retained in the finished film.