Klaus Emil Julius Fuchs (29 December 1911 – 28 January 1988) was a German-British theoretical physicist and atomic spy who in 1950 was convicted of supplying information from the American, British and Canadian atomic bomb research (the Manhattan Project) to the USSR during and shortly after World War II. While at the Los Alamos National Laboratory, Fuchs was responsible for many significant theoretical calculations relating to the first fission weapons and later, the early models of the hydrogen bomb, the first fusion weapon.