  Neutrinoless double decay is a unique probe for lepton number conservation
and neutrino properties. It allows to investigate the Dirac/Majorana nature of
the neutrinos and their absolute mass scale (hierarchy problem) with
unprecedented sensitivity. A number of experiments are presently under
preparation to cover the quasi-degenerate region of the neutrino mass spectrum.
Improved sensitivities are however required to sound the so-called inverted
hierarchy region. This is a real challenge faced by a number of new proposed
projects, based either on large expansions of the present experiments or on new
ideas top improve the technical performance or reduce the background
contributions. A review of the most relevant ongoing experiments is given. The
most relevant parameters contributing to the experimental sensitivity are
discussed and a critical comparison of the future projects is proposed.
