In this article Physicist Alfred O. Nier discusses his work on mass spectrometry for the Manhattan Project. Nier recalls the history of uranium and mass spectrometry. He then talks about his specific work on isolating the fissionable 235 isotope of uranium through mass spectrometry and his work on the use of mass spectrometers to monitor impurities in the process streams of the gaseous diffusion plant at Oak Ridge, Tennessee. The article includes charts and photographs of Nier's work. The article is included in the Journal of Chemical Education commemoration of fifty years of nuclear fission.