Atomic spies or atom spies were people in the United States, the United Kingdom, or 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 so accused actually 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 favored sharing classified information with the world scientific community. This proposal was firmly quashed by the U.S. government.

Atomic spies were motivated by a range of factors. For some, ideology (such as a commitment to communism or other socialist models committed to advancing the interests of the Soviet Union) was the primary reason for their spying. Others were motivated by financial gain, while some may have been coerced or blackmailed into spying. The prospect of playing a role in shaping the outcome of the Cold War may also have been appealing to some. Another large motivational factor was being engrained into the history of the world, an/or being remembered as someone who did something "larger than themselves". Regardless of their specific motivations, each individual played a significant role in the way the Cold War unfolded, and the continuing state of nuclear weapons.

Confirmation about espionage work came from the Venona project, which intercepted and decrypted Soviet intelligence reports sent during (and after) World War II. In 1995, the U.S. declassified its Venona Files which consisted of deciphered 1949 Soviet intelligence communications. These provided clues to the identity of several spies at Los Alamos and elsewhere, some of whom have never been identified. These decryptions prompted the arrest of naturalized British citizen Klaus Fuchs in 1950. Fuchs's confession led to the discovery of spy Harry Gold who served as his Soviet courier. Gold identified spy David Greenglass, a Los Alamos Army-machinist. Greenglass identified his brother-in-law, spy Julius Rosenberg, as his control. The Venona Files corroborated their espionage activities and also revealed others in the network of Soviet spies, including physicist Theodore Hall who also worked at Los Alamos. Some of this information was available to the government during the 1950s trials, but it was not usable in court as it was highly classified. Historians have found that records from Soviet archives, which were briefly opened to researchers after the fall of the Soviet Union, included more information about some spies.

Transcription of declassified Soviet KGB documents by ex-KGB officer Alexander Vassiliev provides additional details about Soviet espionage from 1930 to 1950, including the greater extent of Fuchs, Hall, and Greenglass's contributions. In 2007, spy George Koval, who worked at both Oak Ridge and Los Alamos, was revealed. According to Vassiliev's notebooks, Fuchs provided the Soviet Union the first information on electromagnetic separation of uranium and the primary explosion needed to start the chain reaction, as well as a complete and detailed technical report with the specifications for both fission bombs. Hall provided a report on Los Alamos principle bomb designs and manufacturing, the plutonium implosion model, and identified other scientists working on the bomb. Greenglass supplied information on the preparation of the uranium bomb, calculations pertaining to structural issues with it, and material on producing uranium-235. Fuchs's information corroborated Hall and Greenglass. Koval had access to critical information on dealing with the reactor-produced plutonium's fizzle problem, and how using manufactured polonium corrected the problem. With all the stolen information, Soviet nuclear ability was advanced by several years at least.

Before World War II, the theoretical possibility of nuclear fission resulted in intense discussion among leading physicists world-wide. Scientists from the Soviet Union were later recognized for their contributions to the understanding of a nuclear reality and won several Nobel Prizes. Soviet scientists such as Igor Kurchatov, L. D. Landau, and Kirill Sinelnikov helped establish the idea of, and prove the existence of, a splittable atom. Dwarfed by the Manhattan Project conducted by the US during the war, the significance of the Soviet contributions has been rarely understood or credited outside the field of physics. According to several sources, it was understood on a theoretical level that the atom provided for extremely powerful and novel releases of energy and could possibly be used in the future for military purposes.

In recorded comments, physicists lamented their inability to achieve any kind of practical application from the discoveries. They thought that creation of an atomic weapon was unattainable. According to a United States Congressional joint committee, although the scientists could conceivably have been first to generate a man-made fission reaction, they lacked the ambition, funding, engineering capability, leadership, and ultimately, the capability to do so. The undertaking would be of an unimaginable scale, and the resources required to engineer for such use as a nuclear bomb, and nuclear power were deemed too great to pursue.

At the urging of Albert Einstein and Leo Szilard in their letter of August 2, 1939, the United States – in collaboration with Britain and Canada – recognized the potential significance of an atomic bomb. They embarked in 1942 upon work to achieve a usable device. Estimates suggest that during the quest to create the atomic bomb, an investment of $2 billion, temporary use of 13,000 tons of silver, and 24,000 skilled workers drove the research and development phase of the project. Those skilled workers included the people to maintain and operate the machinery necessary for research. The largest Western facility had five hundred scientists working on the project, as well as a team of fifty to derive the equations for the cascade of neutrons required to drive the reaction. The fledgling equivalent Soviet program was quite different: The program consisted of fifty scientists, and two mathematicians trying to work out the equations for the particle cascade. The research and development of techniques to produce sufficiently enriched uranium and plutonium were beyond the scope and efforts of the Soviet group. The knowledge of techniques and strategies that the Allied programs employed, and which Soviet espionage obtained, may have played a role in the rapid development of the Soviet bomb after the war.

The research and development of methods suitable for doping and separating the highly reactive isotopes needed to create the payload for a nuclear warhead took years and consumed a vast quantity of resources. The United States and Great Britain dedicated their best scientists to this cause and constructed three plants, each with a different isotope-extraction method. The Allied program decided to use gas-phase extraction to obtain the pure uranium necessary for an atomic detonation. Using this method took large quantities of uranium ore and other rare materials, such as graphite, to successfully purify the U-235 isotope. The quantities required for the development were beyond the scope and purview of the Soviet program.