  Neutrinoless double beta decay pops up almost in any extension of the
standard model. It is perhaps the only process, which can unambiguously
determine whether the massive neutrinos are Majorana or Dirac type particles.
In addition from the lifetime of this decay, combined with sufficient knowledge
of the relevant nuclear matrix elements, one can set a constraint involving the
neutrino masses. Furthemore, if one incorporates the recent results of the
neutrino oscillation experiments, one can determine or set a stringent limit on
the neutrino mass scale. In addition one may obtain usefull information
regarding the presence of right handed currents and the right handed neutrino
mass scale. One can also constrain the parameters of supersymmetry and, in
particular, set limits in of R-parity violating couplings as well as get
information about extra dimensions.
