  The predictions for the effective Majorana mass |<m>| in neutrinoless
double-beta decay in the case of 3-neutrino mixing and massive Majorana
neutrinos are reviewed. The physics potential of the experiments, searching for
neutrinoless double-beta decay and having sensitivity to $|<m>| \gtap 0.01$ eV,
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 discussed.
