  Neutrinoless double beta decay is the most powerful tool to probe not only
for Majorana neutrino masses but for lepton number violating physics in
general. We discuss relations between lepton number violation, double beta
decay and neutrino mass, review a general Lorentz invariant parametrization of
the double beta decay rate, highlight a number of different new physics models
showing how different mechanisms can trigger double beta decay, and finally
discuss possibilities to discriminate and test these models and mechanisms in
complementary experiments.
