UFO: Alien Invasion is a futuristic, squad-based and turn-based strategy game set in 2084, heavily inspired by X-COM, but not a sequel or a remake. The game uses a heavily modified version of the Quake II engine. The player controls the secret organization PHALANX and has to construct bases around the globe, and hire soldiers, workers and scientists to fight off an alien invasion. Base construction and the tracking of alien threats takes place on a world map (Geoscape mode), and the fighting is turn-based and done on a 3D map where the player controls a squad of soldiers (Tactical mode). Major tasks include protecting civilians, and capturing aliens and their technology in order to get money and better weapons.
Multiplayer is supported over LAN and internet, with head-to-head sessions, but also cooperative games with a maximum of 6 teams playing, against each other, or AI-controlled opponents. The game is free software, licensed under the GNU General Public License.