Despite her physical absence from Berlin, Meitner still corresponded with her colleagues by post. She set up home in Gothenborg, and worked in Manne Siegbahn's Nobel Institute for Physics. Soon after her arrival, she met her favourite nephew, Otto Frisch, who was also a nuclear physicist, but who had fled to Copenhagen rather than Sweden.

Although Hahn later downplayed Meitner's contribution to their research, (a denial Meitner never overcame, despite Niels Bohr's efforts to set the record straight) Meitner's Berlin colleagues were at a loss to proceed in her absence, as she was the driving impetus in their work, especially in the accompanying theory and logic. At her instruction, Hahn bombarded uranium with neutrons, and he found that a strange radioactive homologue of Barium was formed. This could not be explained by Hahn, Strassman or any other contemporary physicist.

In October 1938, when Hahn wrote to Meitner telling her of the result, Otto Frisch was staying nearby. Both Meitner and Frisch enjoyed the recent snowfalls, Frisch liked to ski, and Meitner was no doubt reminded of her homeland, and the two regularly walked and discussed science together. Meitner showed Frisch the letter, and immediately realised that the Uranium nucleus had split, rather than merely having small particles chipped off it.

Meitner realised that if one of the two fragments was Barium, the other was Krypton, and there should also be an accompanying release of several neutrons and a large amount of energy. Meitner was the first in the world to explain what had happened, and the first to realise the massive release of energy that takes place (200 MeV). She and Frisch were able to calculate this using Einstein's famous equation for mass-energy equivalence: E = mc².

Perhaps fundamental to this new understanding of what Meitner named 'fission' (again a scientific first) after the equivalent cell division in biology (from the latin word meaning to cleave), was the water drop model of the nucleus. Some sources say that this model was suggested to Meitner by Niels Bohr, whereas others say that Meitner created it herself and Bohr expanded on it later (some also suggest that George Gamow was the first…). In any case, Meitner was the first to use it in any practical way.

Frisch rushed back to Copenhagen to tell Niels Bohr of Meitner's discovery. Bohr said of it 'Oh! What idiots we all have been! Oh but this wonderful!' and at once, many years of frustrated theory, paradoxes and inadequate explanation were put to rest.

Soon, Frisch and Bohr proved Meitner's ideas experimentally in Copenhagen, and Frisch and Meitner published a paper explaining newly-titled fission which was published in the German scientific magazine Naturewissenschaften (Nature) on the 6^th January 1939. Immediately, these results were confirmed around the world. In 1946, Hahn was awarded the Nobel prize for Physics for his research into fission, but Meitner was ignored.

Following her success and prominence, Meitner was invited to join the Manhattan Project, building an atomic weapon. Her work led directly to the possibility of nuclear weapons, but Meitner would have no part in building a weapon of such destructive force. She went to great lengths to distance herself from the negative possibilities her discoveries created.

After the war, Meitner became a visiting professor at the Catholic University in Washington D.C., where she worked for a year before retiring in 1947. Despite her 'retirement', she continued to work, first at the Swedish Royal Academy for Engineering Sciences, then at the Swedish Atomic Energy Committee. Her work on isotopes produced in cyclotrons is also significant.

In 1958, She moved to Cambridge, England, and fully retired in 1960. She passed away in 1968. Over her brilliant and prolific career, she published over 135 scientific papers, and was involved in the cutting edge of nuclear research for over 55 years.