  The compelling experimental evidences for oscillations of solar, reactor,
atmospheric and accelerator neutrinos imply the existence of 3-neutrino mixing
in the weak charged lepton current. The current data on the 3-neutrino mixing
parameters are summarised and the phenomenology of 3-$\nu$ mixing is reviewed.
The properties of massive Majorana neutrinos and of their various possible
couplings are discussed in detail. Two models of neutrino mass generation with
massive Majorana neutrinos - the type I see-saw and the Higgs triplet model,
are briefly reviewed. The problem of determining the nature - Dirac or
Majorana, of massive neutrinos is considered. The predictions for the effective
Majorana mass $\meff$ in neutrinoless double beta ($\betabeta-$) decay in the
case of 3-neutrino mixing and massive Majorana neutrinos are summarised. The
physics potential of the experiments, searching for $\betabeta-$decay for
providing information on the type of the neutrino mass spectrum, on the
absolute scale of neutrino masses and on the Majorana CP-violation phases in
the PMNS neutrino mixing matrix, is also briefly discussed. The opened
questions and the main goals of future research in the field of neutrino
physics are outlined.
