In the Standard Model of Elementary Particles the neutrino ν is assumed to be not the same particle as the anti-neutrino  ν, i.e it is a Dirac particle. In many extensions to the Standard Model aiming to the Grand Unification of particle interactions and in theories of particle mass generation, the neutrino and the anti-neutrino turn out to be the same particle, i.e. the neutrino is a Majorana particle. The only available experimental probe to asses the actual nature of the neutrino is the search for the neutrinoless double beta decay.