The Capital Wasteland encompasses the remnants of Washington, D.C. and its surrounding area after the nuclear holocaust of 2077 that ignites the Fallout universe and turns United States into the Wasteland. It is a bleak, desert-like, and highly irradiated terrain that has little to no means of supporting natural vegetation and only a few species of feral creatures have been able to withstand these harsh conditions. All water supplies throughout the Capital Wasteland were exposed to the nuclear fallout causing them to be radioactive, including the Potomac River which runs through the middle of the Capital Wasteland from the northwest to southeast corners. The water is still potable but the consumer may contract radiation poisoning/sickness if he/she drinks large volumes of it. It wasn't until Project Purity went online that purified water became available in large quantities. This area is estimated to be 16 square miles in size and contains over 100 unique locations, landmarks, cities and settlements.
- Brotherhood of Steel - Located in the Citadel, the Brotherhood of Steel seeks to help the inhabitants of the Capital Wasteland by ridding D.C. of Super Mutants. They try to be the protectors of the Capital Wasteland and frequently halt raiders, mutants, and the enclave before they can attack one of the Wastelands many communities.
- Brotherhood Outcasts - A group of former Brotherhood of Steel members who abandoned the chapter due to ideological differences.
- Enclave - Remnants of the pre-war government, the Enclave are considered to be one of the main antagonists of the game. They are very similar to the Brotherhood of Steel, but are selfish and have lost sight of protecting the people of the Capitol Wasteland. Raven Rock is the Enclave's base of operations.
- Super Mutants - Mutated humans that roam the Wasteland and bring prisoners to Vault 87 so they can transform them into Super Mutants or savagely kill them. They resemble muscular, green, brutes much like the hulk and come in several different variations.
- Raiders - Typical organization of bandits and crooks. Raiders can not be reasoned with and they always result to trying to kill people and scavenge their bodies for better supplies. They are found everywhere but have a noticeably large settlement in Evergreen Mills.
- Ghouls - Ghouls are humans that have been exposed to enormous amounts of radiation which evolved them into ghouls. There are two types of ghouls in Fallout. Regular ghouls are people who are just badly disfigured due to their exposure to heavy amounts of radiation and have become outcasts in society. Regular ghouls mostly live in the Museum of Natural History, now called the underworld. Feral ghouls are practically mindless zombies who live in the dark subways.
- Slavers - Slavers are ruthless bounty hunters who capture specific targets and sell them as slaves, followers, body guards, or what ever else a buyer would need them for. Slavers keep their captives and sell them at Paradise Falls.