  A brief overview is given of theoretical analyses with neutrinoless double
beta decay experiments. Theoretical bounds on the ``observable'', <m>_betabeta,
are presented. By using experimental bounds on <m>_betabeta, allowed regions
are obtained on the m_l-cos{2theta_12} plane, where m_l stands for the lightest
neutrino mass. It is shown that Majorana neutrinos can be excluded by combining
possible results of future neutrinoless double beta decay and {}^3H beta decay
experiments. A possibility to constrain one of two Majorana phases is discussed
also.
