When X-COM: UFO Defense (Europe: “UFO: Enemy Unknown”) arrived on the scene for very old DOS computers, the game brought a great deal of innovation to computer gaming. Even the first game includes multi-layered gameplay and plot elements. The X-COM series is known for it's mix of economic strategy in “Geoscape” (base building and management, which includes the research aspect of the game) mode, and action-point turn-based strategy where the player controls individual characters in skirmishes (such as at crash sites and invasion sites). The ability to research weapons and technology for your bases and characters also adds to gameplay greatly. Because of the popularity of the first two X-COM games, X-COM: UFO Defense was eventually ported to Sony PlayStation®.
In addition to traditional X-COM Games, there are some other games in the X-COM series that didn't do too well, such as a multiplayer game that uses e-mail as a networking method, and a first-person shooter game, X-COM: Enforcer--the first person shooter game is actually a remnant of a failed game, X-COM: Alliance, that was intended to be a realtime 3D sequel to X-COM that would integrate first-person control into the strategy of skirmish scenarios.