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.

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
Little is known about the aliens' biology, but we can assume several things based on observations. The aliens' large, developed craniums house a developed brain. When engaging the aliens for the first time in your holding cell, Somah tells you to hit their heads. Therefore, it would seem that their heads are largely unprotected. They are very skinny, implying centuries of physical devolution. Their reliance on technology supports this, as without it, they are easily defeated. They have no visible teeth, possibly limiting their food sources to soft, squishy invertebrates like worm and squid-like creatures. While their hands lack thumbs completely, their fingers are so long they can wrap around objects, eliminating the need for thumbs. Curiously, their feet appear to be similar in shape to human feet, implying that conditions on their homeworld are similar to those of Earth (that is, before the war). Listening to the Alien Captive Recordings, we can detect patterns in their language. In addition, Holly Barrisford is capable of reproducing the sounds they make. This shows that they have vocal chords similar to ours, and that they aren't capable of communicating through use of telekinesis. They cannot be lifted off the ground by the player. This either shows that they are far heavier than their size and shape imply, or that they are being influenced by some sort of artificial gravity system.

Combat

 * Normal aliens are very frail and do not have much health. However, they are armed with powerful energy weapons and can quickly chip away the player's HP.
 * Aliens will occasionally have on 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. It is advisable to use weapons such as the Deathclaw Gauntlet and the Microwave Emitter in those cases, to bypass the massive armor bonus.

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 a special encounter 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).

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 Mothership Zeta's Bridge is captured by Lone Wanderer.
 * "Paladine" crashed craft.
 * Firelance Event craft - Identical vehicle to Recon Craft Theta, destroyed by unknown cause whilst near the Lone Wanderer.

Other

 * Alien Homing Beacon - A beacon teleportation sent by a mothership to return quickly
 * Alien Biogel - Ingestible and curative gelatin-like liquid
 * Alien Epoxy - Alien repairing agent
 * Alien Turret - Turret which fire electric projectiles
 * Death Ray - Powerful canon equip by 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
Crashed alien ships and alien dead bodies appear in Fallout and Fallout 3. The Alien Blaster appears in all three games. Aliens themselves appear and abduct the player in the Fallout 3 add-on, Mothership Zeta.