Neutrinos: Majorana or Dirac?

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.