  Whether there exist elementary particles having Majorana nature is one the
fundamental open question that has persisted since the 1930s. The only
practical experiments to test the Majorana nature of neutrinos is the search
for neutrinoless double-beta decay, which has been a major challenge for
nuclear and particle physicists. In the 2000's, a number of experiments using
advanced technologies have been planned, and some of which have already
achieved significant improvements in the search sensitivity. In this article,
the current status of the neutrinoless double-beta decay searches are
summarized, reviewing the progress of KamLAND-Zen, which recorded the world
best sensitivity in the effective Majorana neutrino mass limit.
