Neutrino mixing and the nature of massive neutrinos

The phenomenology of 3-neutrino mixing is briefly reviewed. The problem of determination of the nature - Dirac or Majorana, of massive neutrinos is discussed. The physics potential of experiments, searching for (beta beta)(0 nu)-decay, for providing information on the type of nu-mass spectrum, absolute scale of nu- masses and on the Majorana CP-violation phases in the PMNS neutrino mixing is reviewed. Possible effects of Majorana phases on the predictions for the rates of LFV decays mu -> e + gamma, etc. in a class of SUSY theories with see-saw mechanism of nu-mass generation, are also briefly considered.