Various locations in the post-nuclear United States of America form the scenery in which the events of the Fallout series occur. Many of which have multiple diverse environments where anything can happen. Such as a special encounter, or just an outright skirmish against raiders. The environments and places have widely been changed over the course of the many games, as the settings have always been in different places, and always brought new surprises with them.
In the Capital Wasteland, there are 22 settlements, ranging in size from Rivet City, with a population of 43 named characters and 7 Stores, to Girdershade: 2 people and no merchants. Apart from Rivet City, there are 4 other major settlements: Megaton, which would be probably the first settlement the Lone Wanderer encounters after escaping the vault; Underworld, the city inside downtown DC full of ghouls; Tenpenny Tower, a former luxury hotel that Allistair Tenpenny lets people in for the right price-ghouls not apply; and the Citadel, home of the Brotherhood of Steel. besides those, there is the Republic of Dave, Canterbury Commons, Oasis, Paradise Falls, Temple of the Union, Little Lamplight, Arefu, Andale, and the Reilly's Rangers Compound are all settlements of a medium size, and all have related quests. Vault 101 and 112 both are also populated: Vault 101 is where the story starts, yet is closed after you exit for the first time, is reopened during the the Trouble on the Homefront quest, and after that, it is once again sealed, but this time for good. However there is evidence of an inevitable rebellion as Butch is found in Rivet City in the Muddy Rudder. Vault 112, home of the Tranquility Lane simulator, is populated only by RoboBrains and, briefly, the Lone wanderer's father, James.
There were a total of 122 Vaults that were known to the public, and this number may not include any of the secret Vaults that the pre-War American government could have constructed for private purposes of protecting high-ranking officials. Most of the Vaults were actually created for testing some sort of social, or emotional flaw and how they would react to it, and only about 17 of the Vaults were actually meant for people to live normal lives in the Vault, and not to be exposed to carnivorous plants (Vault 22), or the door not meant to close all the way, thus exposing the residents to lethal amounts of radiation (Vault 12).