Race X is a hostile alien race featured in the computer game series Half-Life by Valve Software. They appear only in the Opposing Force expansion pack, and very little is revealed about them. Race X aliens appear late in the timeline of Half-Life, only a few hours before Gordon Freeman has teleported to Xen (an event that takes place in both Half-Life and Opposing Force). No explanation is given for the appearance of Race X aliens on Earth in-game, though the official strategy guide states that the race's intentions are to invade the planet to acquire its natural resources.