  Neutrinoless double beta decay is a promising test for lepton number
violating physics beyond the standard model of particle physics. There is a
deep connection between this decay and the phenomenon of neutrino masses. In
particular, we will discuss the relation between neutrinoless double beta decay
and Majorana neutrino masses provided by the so-called Schechter--Valle theorem
in a quantitative way. Furthermore, we will present an experimental cross check
to discriminate neutrinoless double beta decay from unknown nuclear background
using only one isotope, i.e., within one experiment.
