Theodore Alvin Hall was an American physicist and an atomic spy for the Soviet Union, who, during his work on United States efforts to develop the first and second atomic bombs during World War II, gave a detailed description of the "Fat Man" plutonium bomb, and of several processes for purifying plutonium, to Soviet intelligence.

Atomic spies or atom spies were people in the United States, the United Kingdom, and Canada who are known to have illicitly given information about nuclear weapons production or design to the Soviet Union during World War II and the early Cold War. Exactly what was given, and whether everyone on the list gave it, are still matters of some scholarly dispute. In some cases, some of the arrested suspects or government witnesses had given strong testimonies or confessions which they recanted later or said were fabricated. Their work constitutes the most publicly well-known and well-documented case of nuclear espionage in the history of nuclear weapons. At the same time, numerous nuclear scientists wanted to share the information with the world scientific community, but this proposal was firmly quashed by the United States government. It is worth noting that many scientists who worked on the Manhattan Project were deeply conflicted about the ethical implications of their work, and some were actively opposed to the use of nuclear weapons.