  Are neutrinos with definite masses Majorana or Dirac particles? This is one
of the most fundamental problem of the modern neutrino physics. The solution of
this problem could be crucial for understanding of the origin of small neutrino
masses. We will review here basic arguments in favor of the Majorana nature of
massive neutrinos. The phenomenological theory of $0\nu\beta\beta$-decay is
briefly discussed and recent experimental data and sensitivity of future
experiments are presented.
