The Montreal Laboratory was a program established by the National Research Council of Canada in the 1940s during World War II to undertake nuclear research in collaboration with the United Kingdom, and to absorb some of the scientists and work of the Tube Alloys nuclear project in Britain. It became part of the Manhattan Project, and designed and built some of the world's first nuclear reactors.

After the Fall of France, some French scientists escaped to Britain with their stock of heavy water. They were temporarily installed in the Cavendish Laboratory at the University of Cambridge, where they worked on reactor design. The MAUD Committee was uncertain whether this was relevant to the main task of Tube Alloys, that of building an atomic bomb, although there remained a possibility that a reactor could be used to breed plutonium, which might be used in one. It therefore recommended that they be relocated to the United States, and co-located with the Manhattan Project's reactor effort. Due to American concerns about security (many of the scientists were foreign nationals) and patent claims by the French scientists and Imperial Chemical Industries (ICI), it was decided to relocate them to Canada instead.

The Canadian government agreed to the proposal, and the Montreal Laboratory was established in a house belonging to McGill University; it moved to permanent accommodation at the Université de Montréal in March 1943. The first eight laboratory staff arrived in Montreal at the end of 1942. These were Bertrand Goldschmidt and Pierre Auger from France, George Placzek from Czechoslovakia, S. G. Bauer from Switzerland, Friedrich Paneth and Hans von Halban from Austria, and R. E. Newell and F. R. Jackson from Britain. The Canadian contingent included George Volkoff, Bernice Weldon Sargent and George Laurence, and promising young Canadian scientists such as J. Carson Mark, Phil Wallace and Leo Yaffe.

Although Canada was a major source of uranium ore and heavy water, these were controlled by the Americans. Anglo-American cooperation broke down, denying the Montreal Laboratory scientists access to the materials they needed to build a reactor. In 1943, the Quebec Agreement merged Tube Alloys with the American Manhattan Project. The Americans agreed to help build the reactor. Scientists who were not British subjects left, and John Cockcroft became the new director of the Montreal Laboratory in May 1944. The Chalk River Laboratories opened in 1944, and the Montreal Laboratory was closed in July 1946. Two reactors were built at Chalk River. The small ZEEP went critical on 5 September 1945, and the larger NRX on 21 July 1947. NRX was for a time the most powerful research reactor in the world.

Canada has a long history of involvement with nuclear research, dating back to the pioneering work of Ernest Rutherford at McGill University in 1899. In 1940, George Laurence of the National Research Council (NRC) began experiments in Ottawa to measure neutron capture and nuclear fission in uranium to demonstrate the feasibility of a nuclear reactor. For that purpose, he obtained 450 kilograms (990 lb) of uranium dioxide in paper bags from the Eldorado Mine at Port Radium in the Northwest Territories. For a neutron moderator, he used carbon in the form of petroleum coke. This was placed with the bags of uranium oxide in a large wooden bin lined with paraffin wax, another neutron moderator. A neutron source was added and a Geiger counter used to measure radioactivity.

The experiments continued in 1942, but were ultimately unsuccessful; the problems posed by impurities in the coke and uranium oxide had not been fully appreciated, and as a result too many neutrons were captured. But Laurence's efforts attracted some attention, and in the summer of 1940 he was visited by R. H. Fowler, the British scientific liaison officer in Canada. This was followed by a visit from John Cockcroft of the British Tizard Mission to the United States in the autumn. They brought news of the similar research being carried out under the supervision of the MAUD Committee in Britain and the National Defense Research Committee (NDRC) in the United States.

Fowler became the channel of communication between the NDRC and its counterparts in Britain and Canada. Through him, Laurence obtained an introduction to Lyman J. Briggs, the chairman of the NDRC's S-1 Uranium Committee, who supplied copies of American studies. On returning to England, Cockcroft arranged through Lord Melchett for Laurence to receive a $5,000 grant to continue his research. This payment was made by Imperial Chemical Industries (ICI) through a Canadian subsidiary. It had the desired side effect of impressing the Canadian authorities with the importance of Laurence's work.

Laurence had chosen to use carbon instead of heavy water because it was cheaper and more readily available. A team of scientists in France that included Hans von Halban, Lew Kowarski, and Francis Perrin had been conducting similar experiments since 1939. By 1940, they had decided to use heavy water as a moderator, and through the French Minister of Armaments obtained about 185 kilograms (408 lb) from the Norsk Hydro hydroelectric station at Vemork in Norway. After the Fall of France, they had escaped to Britain with their stock of heavy water. They were temporarily installed in the Cavendish Laboratory at the University of Cambridge but, believing that Britain would soon fall as well, were eager to relocate to the United States or Canada.