  Neutrinoless double beta decay belongs to the most sensitive tools for the
search of new physics beyond the standard model. The recent half life limit of
the Heidelberg-Moscow experiment implies restrictive bounds on the absolute
mass scale in the neutrino sector. Possible improvements by the GENIUS project
provide a unique possibility to reconstruct the neutrino mass spectrum. Further
constraints on new interactions in the neutrino sector are given in a
model-independent way. Consequences for neutrino anomalies and theories beyond
the standard model such as left-right symmetric models, R-parity violating SUSY
and leptoquarks are discussed. The potential of double beta decay experiments
in the search for WIMP dark matter is reviewed.
