The individuals known collectively as the Atom Spies succeeded in passing intelligence about the top secret British and American atomic programmes to representatives of Soviet intelligence organisations. The information that they provided made a significant impact upon Soviet atomic research, allowing the USSR's scientists to copy the technologies developed by the Manhattan Engineering District (MED), and accelerating the arrival of the Soviet atomic bomb by at least a year. This dissertation examines the various interactions between these Atom Spies and the counter-intelligence organisations in Britain and the United States, using a comparative framework. The first chapter examines how these individuals were cleared to work on top secret atomic research and draws broader conclusions about the security policies employed in each country. Moving into the postwar period, the second chapter involves discussion of how the defection of Soviet cipher clerk lgor Gouzenko and the intensification of the Cold War affected both atomic security and collaboration between Britain and the United States. The middle section analyses counterespionage operations against the Atom Spies in Britain and America, emphasising the role of Signals Intelligence and comparing the investigative strategies employed by MI5 and the FBI. The penultimate chapter investigates the effect of the Atom Spy cases on atomic security, and includes a detailed examination of the case of Bruno Pontecorvo, a British-based atomic scientist who defected to the Soviet Union in 1950. Lastly, some of the broader consequences of the Atom Spy cases are discussed; as shall emerge, the exposure of the Atom Spies played a major role in the postwar evolution of both the atomic and intelligence relationship between Britain and the United States, as well as contributing to the American debate over the need to develop a Hydrogen bomb.