Aliens is a 1986 science fiction action film written and directed by James Cameron. It is the sequel to the 1979 science fiction horror film Alien, and the second film in the Alien franchise. Set in the far future, it stars Sigourney Weaver as Ellen Ripley, the sole survivor of an alien attack on her ship. When communications are lost with a human colony on the moon where her crew first encountered the alien creatures, Ripley agrees to return to the site with a unit of Colonial Marines to investigate. Michael Biehn, Paul Reiser, Lance Henriksen, and Carrie Henn are featured in supporting roles.

Despite the success of Alien, its sequel took years to develop due to lawsuits, a lack of enthusiasm from 20th Century Fox, and repeated management changes. Although relatively inexperienced, Cameron was hired to write a story for Aliens in 1983 on the strength of his scripts for The Terminator (1984) and Rambo: First Blood Part II (1985). The project stalled again until new Fox executive Lawrence Gordon pursued a sequel. On an approximately $18.5 million budget, Aliens began principal photography in September 1985 and concluded in January 1986. The film's development was tumultuous and rife with conflicts between Cameron and the British crew at Pinewood Studios. The difficult shoot affected the composer, James Horner, who was given little time to record the music.

Aliens was released on July 18, 1986, to critical acclaim. Reviewers praised its action, but some criticized the intensity of some scenes. Weaver's performance garnered consistent praise along with those of Bill Paxton and Jenette Goldstein. The film received several awards and nominations, including an Academy Award nomination for Best Actress for Weaver at a time when the science-fiction genre was generally overlooked. It earned $131.1–183.3 million during its theatrical run, making it one of the highest-grossing films of 1986 worldwide.

Aliens is now considered among the greatest films of the 1980s, and among the best science fiction, action, and sequel films ever made, arguably equal to or better than Alien. It is credited with expanding the franchise's scope with additions to the series' backstory and factions such as the Colonial Marines. It inspired a variety of merchandise, including video games, comic books and toys. It was followed by two sequels: Alien 3 (1992) and Alien Resurrection (1997) and a prequel film, Alien: Romulus (2024).