Betrayal is a recurring theme in the Fallout series of games.

Generally, betrayal is the act of "producing or exposing to an enemy in disloyalty and/or treachery." Many examples of this can be found in the Fallout series.

Fallout

• Perhaps the most significant example, The Overseer betrays the Vault Dweller by refusing to readmit him/her to Vault 13 upon the completion of the main quest.
• Alternatively, the player can betray the location of Vault 13 to the Master.

Fallout 2

• The Enclave Company Control squad, led by Granite, will betray the Enclave by fighting along the Chosen One's side (Speech check is required).
  • Likewise, Tom Murray and Charles Curling would also betray the Enclave with a Speech check.
• The Chosen One can betray John Bishop/Roger Westin by killing the men, even if they promised rich rewards by killing their opponent.
• Torr Buckner can be told that he needs to go while the Chosen One can speak to Dunton brothers.
• The Chosen One can be told to deliver a message to Modoc from Vegeir, but he does not need to deliver it. Instead, he can attack the town with Modoc's people.

Fallout Tactics

• The Calculator was intended to aid the survivors in rebuilding America, instead it worked for their extermination.
• General Simon Barnaky, an early mentor to the player character, aids the Calculator in waging war against the Midwestern Brotherhood of Steel, although how willing he was in this is a matter of interpretation.

Van Buren

• In the Betray Sharon and Betray Trisha quests.
• The Twisted Hairs were betrayed by Caesar and Vulpes Inculta after an alliance was broken by the latter, resulting in the former being enslaved and conquered. Anyone who resisted was crucified along the I-40.

Fallout 3

• Anna Holt betrays Dr. Li and her colleagues when she goes to work for the Enclave instead.
• With Broken Steel installed, The Lone Wanderer can choose to destroy The Citadel for whatever reason, and thus betraying his/her allies in the Brotherhood of Steel.
• The Lone Wanderer can betray Sydney in the Stealing Independence quest.
• The Lone Wanderer can betray Desmond Lockheart in the Point Lookout add on, and then gets betrayed himself by Professor Calvert.
• Mister Crowley was betrayed by Dukov, according to Trapped, trapped, trapped.
• John Henry Eden promises to help the people of the wasteland, but plans to infect them with the Modified FEV virus, thereby killing them.

Fallout: New Vegas

• Jeannie May Crawford betrays Craig Boone by selling his wife, Carla and Carla's unborn child, to the Legion as a slave.
• In the beginning, Benny has betrayed the Great Khans after together capturing and ostensibly killing the Courier, by taking everything and running back to the New Vegas Strip.
• In the Lonesome Road add-on, Ulysses has experienced much betrayal, from Caesar's Legion and the Courier, respectively.
• The Courier can betray Stone by turning him in after receiving the Hydras from him.
• The Courier can betray Eddie by turning him in to the NCR while working for him in the I Fought the Law quest.
• Karl, being sent as a representative from the Legion, betrays the Great Khans.
• The Omertas have only one rule: Never betray the "family".
• Betrayal is an omnipresent theme in the history of Vault 11.
• Barton Thorn tries to betray the Courier in the unmarked quest Barton the Fink.
• In the Dead Money add-on, the Courier learns that Dean Domino and Vera Keyes conspired to betray Frederick Sinclair and rob the Sierra Madre Casino's vault.
• The Courier can betray Robert House.
• In Dead Money, Dean Domino may betray the Courier depending on interactions with him earlier in the add-on.
• In the quest There Stands the Grass the Courier can betray Keely at the end of the quest.
• In a random encounter outside Nipton, Jacklyn will betray Tomas for his Sunset Sarsaparilla star bottle caps and if Jacklyn survives the showdown, depending on dialogue options, she will also betray the Courier.