Spectrum of neutrino masses and their nature in the light of present and future experiments

The present experimental data on neutrino oscillations, neutrinoless double beta decay and tritium beta decay are collected together and possible mass ranges for Dirac and Majorana neutrinos are found. Four future experimental situations are investigated: both decay experiments give only upper bounds, one of them gives a positive result ($|< m_\nu > | \neq 0$ or $m_\beta \neq 0$), or finally both effective neutrino masses are different from zero ($|< m_\nu > | \neq 0$ and $m_\beta \neq 0$). Each scenario gives new information on neutrino masses and nature but only the last has a chance to resolve the problem and give some additional information on CP violation in the lepton sector.