In the original formulation of the standard model, presented in Chapters 7, 8 and 9, neutrinos are massless particles. This feature is well motivated by direct experimental upper bounds on neutrino masses: $$ m_{\nu _e } \leqslant {\mathbf{ }}3{\mathbf{ }}eV;{\mathbf{ }}m_{\nu _\mu } \leqslant {\mathbf{ }}0.19{\mathbf{ }}MeV;{\mathbf{ }}m_{\nu _\tau } \leqslant 18.2{\mathbf{ }}MeV; $$ (12.1) and even if observations indicate that neutrino masses are in fact non-zero, the approximation mv ≪ mf, where f is any fermion in the standard model spectrum, is extremely good for most applications. In view of experimental results, however, it is interesting to study the possible ways neutrino mass terms can be consistently introduced.