Alien

While there always was some evidence of the existence of alien life forms, it is very scarce. Until the events of the Fallout 3 add-on Mothership Zeta, there was no record of direct contact between humanity and the creatures. Despite this, it is apparent with evidence found on Zeta, that the aliens have quite possibly been watching Earth and Humanity evolve for nearly a thousand years.

The Brotherhood of Steel and Shi have encountered the bodies of alien life forms or their spaceships, as did the pre-War United States government, which kept its alien specimens mostly in Area 51. By extension, the Enclave and Brotherhood Outcasts also have access to alien technology; Alien Power Cells can be found at Fort Independence and in the armory of the Enclave's Mobile Base Crawler. They can also be found in the Citadel's armory (if you choose to destroy the Citadel during Who Dares Wins).

Finally being encountered in Mothership Zeta, they are small, greenish humanoids that are presented as malevolent creatures that seem to view humanity with contempt and will gladly vivisect their test subjects or keep them in cryogenic storage after torturing them for their scientific pursuits. The aliens themselves seem to have their own language and cannot be understood by the player in any way.

Biology
When engaging the aliens for the first time in your holding cell, Somah tells you to hit their heads, implying that this area is largely unprotected. They are very skinny, implying reliance on technology, without which they are easily defeated. The aliens have no visible teeth, possibly limiting food sources. While their hands lack thumbs completely, the fingers are so long they can wrap around objects, potentially eliminating the need for thumbs. The design of the fingers is reminiscent of the iconic Martians, seen in the War of the Worlds movies. Their feet appear to be similar in shape to human feet.

Language
The Aliens speak with clicks, high-pitched squeaks and wails. The sounds can sound strange and incomprehensible, but patterns may be detected in their language, and Holly Barrisford is capable of reproducing the sounds they make - showing that they have vocal chords similar to ours. This implies they do not communicate telepathically. Strangely, the alien distress signal speaks in a language unique to that of the mothership - a kind of warble, suggesting multiple alien languages, such as for example, English to French. It is also possible the signal from Theta is distorted by the machinery, or it is some form of code.

Combat

 * Normal aliens are very frail and do not have much health. However, they are armed with powerful energy weapons and can quickly kill the player.
 * Aliens will occasionally wear helmets, even inside the ship. These do not seem to have an effect on damage resistance and appear to be merely cosmetic.
 * Inertia Suppression Fields are sometimes employed by aliens and significantly increase damage resistance; the amount of shots needed to kill a shielded alien tends to at least quadruple. However, the Inertia Suppression Fields don't protect against after-effects such as Jingwei's Shocksword's shock damage or the Flamer's burning damage.
 * Aliens tend to have drones with them in some areas which heightens their threat level as the Guardian Drone's weapon packs a very powerful punch. It would be wise to prioritize the Guardian Drones first, then the aliens.

Crashed ships
Four alien ships are known to have crashed in the United States:
 * An alien ship in special encounter in California (Fallout).
 * Recon Craft Theta in Capital Wasteland (Fallout 3).
 * Another alien ship can explode over the head of the Lone Wanderer in the Capital Wasteland during the Firelance event although the cause of the catastrophe is unknown (Fallout 3).
 * At the Citadel, there is information on a terminal (in the A-Ring) about another ship that crashed in a heavily wooded area that the government was not able to reach (Fallout 3). However, this could be referring to the Recon Craft Theta, as it can be seen that before the fallout, a large amount of trees occupied the surrounding areas of the ship.

Weapons
Firearms:
 * Alien Atomizer
 * Alien Blaster (Two exist)
 * Alien Disintegrator
 * Atomic Pulverizer
 * Captain's Sidearm
 * Destabilizer
 * Drone Cannon
 * Drone Cannon Ex-B
 * Firelance

Melee weapons:
 * Electro-Suppressor
 * Shock Baton

Ammo:
 * Alien Power Cell
 * Alien Power Module

Robots and computers

 * Experimental Weapons Drone
 * Guardian Drone
 * Support Drone
 * According to its own records, the Skynet computer was created based partly on alien technology. (Fallout 2)

Vehicles

 * Alien Ship crash site in California - A small saucer that housed 2 pilots.
 * Recon Craft Theta - Small alien vehicle, found in Capital Wasteland. Used by 1 pilot.
 * Mothership Zeta - Large alien vehicle located in space near Earth dedicated to study of humanity.
 * The Second Mothership - A ship of the same design as Mothership Zeta, it comes right after Zeta's Bridge is captured by the Lone Wanderer.
 * "Paladine" crashed craft.
 * Firelance Event craft - Identical vehicle to Recon Craft Theta, destroyed by unknown cause whilst near the Lone Wanderer.

Other

 * Abominations - Beings created from humans as a result of Alien experiment.
 * Alien Biogel - Ingestible and curative gelatin-like liquid (Stated that they pour/rub it into injuries)
 * Alien Epoxy - Alien repairing agent
 * Alien Homing Beacon - A beacon teleportation sent by a mothership to return quickly
 * Alien Squid Food - A strange substance the aliens experiment on that can be used as food.
 * Alien Turret - Turret which fire electric projectiles
 * Death Ray - Powerful cannon equipped on mothership
 * Drone Control Device - Remote control for drones
 * Force Field Generator - Used for protective cover or blocking a passage
 * Healing Archway - Care generator in mothership
 * Inertia Suppression Field - Personal protection shield technology
 * Other various objects that the Lone Wanderer cannot get

Appearances
Aliens appear in Fallout, Fallout 2, Fallout 3 and Mothership Zeta add-on.