In the standard model, neutrinos can be described by $SU(2)_L×U(1)_Y$ gauge theory and they are introduced as truly massless and neutral particles. But, we faced to some problems, for example solar neutrino puzzle, and realized that these problems can be solved by neutrino oscillation. Neutrino oscillation means that neutrinos are massive, but the standard model does not allow it. So, many people try to describe the existence of neutrino mass until now. The difference between neutrino mass eigenstate and flavor eigenstate causes the oscillation. As an electron neutrino, muon neutrino or tau neutrion propagates, it can be converted to each other. There is an MSW effect, which has a resonance region, for neutrino oscillation in matter.
I examine some methods to describe massive neutrinos as Dirac or Majorana fields. And I applied some methods, Higgs mechanism or Yukawa coupling which generates mass term of several fundamental particles, to the neutrinos. And look around Dirac, Majorana and Dirac-Majorana neutrino mass matrices.
Finally, review the solar neutrino, atmospheric neutrino and LSND experiment result.