The history of the phenomenological theory of neutrino interactions and weak interactions in general, begins with the attempts to understand the physics of nuclear radiation known as β-rays. These highly penetrating and ionizing component of the radiation discovered by Becquerel  in 1896 were subsequently established to be electrons by doing many experiments in which their properties like charge, mass, and energy were studied . Since the energy of these β-ray electrons was found to be in the range of a few MeV, they were believed to be of nuclear origin in the light of the basic structure of the nucleus known at that time . It was assumed that the electrons are emitted in a nuclear process called β-decay in which a nucleus in the initial state goes to a final state by emitting an electron. The energy distribution of the β-ray electrons was found to be continuous lying between me, the mass of the electron, and a maximum energy Emax corresponding to the available energy in the nuclear β-decay, that is, Emax = Ei − E f , where Ei and E f are the energies of the initial and final nuclear states. A typical continuous energy distribution for the electrons from the β-decay of RaE is shown in Figure 1.1 of Chapter 1. It was first thought that the electrons in the β-decay process were emitted with a fixed energy Emax and suffered random losses in their energy due to secondary interactions with nuclear constituents as they traveled through the nucleus before being observed leading to a continuous energy distribution. However, the calorimetric heat measurements performed by Ellis et al.  and confirmed later by Meitner et al.  in the β-decays of RaE, established that the electrons emitted in the process of the nuclear β-decay have an intrinsically continuous energy distribution. The continuous energy distribution of the electrons from the β-decay posed a difficult problem toward its theoretical interpretation in the context of the contemporary model of the nuclear structure and seemed to violate the law of conservation of energy.