  The discovery of neutrino masses through the observation of oscillations
boosted the importance of neutrinoless double beta decay ($0\nu\beta\beta$). In
this paper, we review the main features of this process, underlining its key
role both from the experimental and theoretical point of view. In particular,
we contextualize the $0\nu\beta\beta$ in the panorama of lepton-number
violating processes, also assessing some possible particle physics mechanisms
mediating the process. Since the $0\nu\beta\beta$ existence is correlated with
neutrino masses, we also review the state-of-art of the theoretical
understanding of neutrino masses. In the final part, the status of current
$0\nu\beta\beta$ experiments is presented and the prospects for the future hunt
for $0\nu\beta\beta$ are discussed. Also, experimental data coming from
cosmological surveys are considered and their impact on $0\nu\beta\beta$
expectations is examined.
