UFO: Extraterrestrials is a turn-based strategy game for Windows aimed to be a spiritual sequel to the highly acclaimed X-COM: UFO Defense. Developed by Chaos Concept, the game incorporates a twist of the aliens defeating the humans on Earth. The game takes place on a human-colonized planet named Esperanza, where the alien threat has just arrived. An updated version of the game, UFO: Extraterrestrials Platinum, was released in 2023.

UFO: Enemy Unknown, also known as X-COM: UFO Defense in North America, is a 1994 science fiction strategy video game developed by Mythos Games and MicroProse. It was published by MicroProse for DOS and Amiga computers, the Amiga CD32 console, and the PlayStation. Originally planned by Julian Gollop as a sequel to Mythos Games' 1988 Laser Squad, the game mixes real-time management simulation with turn-based tactics. The player takes the role of commander of X-COM – an international paramilitary and scientific organization secretly defending Earth from an alien invasion. Through the game, the player is tasked with issuing orders to individual X-COM troops in a series of turn-based tactical missions. At strategic scale, the player directs the research and development of new technologies, builds and expands X-COM's bases, manages the organization's finances and personnel, and monitors and responds to UFO activity.