  Nuclear double beta decay provides an extraordinarily broad potential to
search for beyond Standard Model physics, probing already now the TeV scale, on
which new physics should manifest itself. These possibilities are reviewed
here. First, the results of present generation experiments are presented. The
most sensitive one of them - the Heidelberg-Moscow experiment in the Gran Sasso
- probes the electron mass now in the sub eV region and will reach a limit of
$\sim$ 0.1 eV in a few years. Basing to a large extent on the theoretical work
of the Heidelberg Double Beta Group in the last two years, results are obtained
also for SUSY models (R-parity breaking, sneutrino mass), leptoquarks
(leptoquark-Higgs coupling), compositeness, right-handed W boson mass, test of
special relativity and equivalence principle in the neutrino sector and others.
One of the enriched $^{76}$Ge detectors also yields the most stringent limits
for cold dark matter (WIMPs) to date by using raw data. A new Heidelberg
experimental proposal (GENIUS) is described which would allow to increase the
sensitivity for Majorana neutrino masses from the present level of at best 0.1
eV down to 0.01 or even 0.001 eV. It would further, already in a first step
using only 100 kg of natural Ge detectors, cover almost the full MSSM parameter
space for prediction of neutralinos as cold dark matter.
