Theodore Alvin Hall (October 20, 1925 – November 1, 1999) was an American physicist and an atomic spy for the Soviet Union, who, during his work on US efforts to develop the first and second atomic bombs during World War II (the Manhattan Project), gave a detailed description of the "Fat Man" plutonium bomb, and of several processes for purifying plutonium, to Soviet intelligence. His brother, Edward Hall, was a rocket scientist who worked on ICBMs for the United States government.