From the time it was initially proposed, the neutrino has been thought to have a very small or zero mass. Using the amount of energy available in β decay when one subtracted the masses and energies of the decay products from the mass of the original nucleus, one could estimate its mass. In his original suggestion of the existence of the neutrino, Wolfgang Pauli wrote, “The mass of the neutrons [neutrinos] should be of the order of magnitude as the electron mass and in any case not larger than 0.01 times the proton mass” (Pauli 1991, p. 5). When Fermi formulated his theory of β decay, he did not assume a specific mass for the neutrino. He noted, however, that the shape of the β-decay energy spectrum near the end point, or maximum energy, depended on the mass of the neutrino. The shape of the spectrum near the end point for different neutrino masses—zero, small, and large—is shown in Figure 6.1(a). Fermi also remarked that the existing empirical evidence on the energy spectra agreed best with the theoretical curves for a zero-mass neutrino (see Figure 2.1). The shape of the -decay energy spectrum near the end point for different neutrino masses, zero, small, and large (a) (<xref ref-type="bibr" rid="CIT00132">Fermi, 1934b</xref>) (b) (<xref ref-type="bibr" rid="CIT00133">Fermi, 1950</xref>). https://s3-euw1-ap-pe-df-pch-content-public-u.s3.eu-west-1.amazonaws.com/9780429199783/d9d7f6b3-b84f-4140-a573-7684ed8d06ff/content/fig6_1_B.tif" xmlns:xlink="https://www.w3.org/1999/xlink"/>