  Neutrinoless double beta-decay(DBD) is of current interest in
high-sensitivity frontiers of particle physics. The decay is very sensitive to
Majorana neutrinos masses, neutrino CP phases, right-handed weak interactions
and others, which are beyond the standard electro-weal model. DBDs are actually
ultra-rare events, and thus DBD experiments with ultra-high sensitivity are
required. Critical discussions are presented on nuclear and detector
sensitivities for high-sensitivity DBD experiments to study the neutrino masses
in the normal and inverted hierarchies.
