  The search for neutrinoless double beta decay ($0\nu\beta\beta$) is one of
the key experiments for determining unresolved properties of neutrinos.
Experimental observation of $0\nu\beta\beta$ would provide a clear
demonstration of the Majorana nature of neutrinos and a lepton number violating
process in particle physics. This report is a brief review describing the
$0\nu\beta\beta$ process and its significance. The detector technologies used
in the present and proposed $0\nu\beta\beta$ experiments are compared among
their recent experimental results. Moreover, the sensitivities for
$0\nu\beta\beta$ discovery in a number of present and proposed $0\nu\beta\beta$
experiments are introduced.
