  In the last two decades the search for neutrinoless double beta decay has
evolved into one of the highest priorities for understanding neutrinos and the
origin of mass. The main reason for this paradigm shift has been the discovery
of neutrino oscillations, which clearly established the existence of massive
neutrinos. An additional motivation for conducting such searches comes from the
existence of an unconfirmed, but not refuted, claim of evidence for
neutrinoless double decay in $^{76}\text{Ge}$. As a consequence, a new
generation of experiments, employing different detection techniques and
$\beta\beta$ isotopes, is being actively promoted by experimental groups across
the world. In addition, nuclear theorists are making remarkable progress in the
calculation of the neutrinoless double beta decay nuclear matrix elements, thus
eliminating a substantial part of the theoretical uncertainties affecting the
particle physics interpretation of this process. In this report, we review the
main aspects of the double beta decay process and some of the most relevant
experiments. The picture that emerges is one where searching for neutrinoless
double beta decay is recognized to have both far-reaching theoretical
implications and promising prospects for experimental observation in the near
future.
