The interaction between neutrinos and matter is an example of the type classified as weak, and such interactions also allow many elementary particle decays and the beta decay of the nuclei. The development of gauge theories has provided the basis for a complete and consistent account of weak interactions, at the same time displaying their unity with electromagnetism. Gauge theories in this context were first proposed in 1967. Rapid mathematical development took place from 1971, and on this book's publication in 1979, a moderate amount of empirical verification had been achieved.