On the Origin of Majorana Neutrino Masses

In the introductory part we briefly consider basics of neutrino oscillations and convenient phenomenology of neutrino oscillations in vacuum. Main part of this report is dedicated to a discussion of a plausible BSM scenarios of neutrino mass generation, based on assumptions of massless left-handed SM neutrinos and violation of the total lepton number. It is stressed that search for light sterile neutrinos and neutrinoless double $\beta$-decay could provide a crucial tests of this scenarios.