Within two years of its proposal, Fermi’s theory had received two challenges. One of these, which concerned the exact mathematical form of the β-decay interaction, was regarded as an articulation of Fermi’s theory rather than as a new theory; it will be discussed below. The second, though it also involved the mathematical form of the interaction, was regarded as a different theory. The next section explains how the decision between these two competing theories was made. The history will show science as both fallible and reasonable. It will include discordant and incorrect experimental results, an incorrect theory-experiment comparison, and a final decision based on critical discussion and experimental evidence. Neither of these two challenges decreased support for the existence of the neutrino, because both alternatives incorporated the neutrino as well as the conservation laws in their formulation. The neutrino will seldom be mentioned directly. This will be, primarily, a history of Fermi’s theory of weak interactions and of how increasing experimental evidence helped both to articulate the theory and to increase its support. That support provides additional grounds for belief in the neutrino.