  Double beta decay is indispensable to solve the question of the neutrino mass
matrix together with $\nu$ oscillation experiments. The most sensitive
experiment since eight years - the Heidelberg-Moscow experiment in Gran-Sasso -
already now, with the experimental limit of < m_\nu > < 0.26 eV excludes
degenerate $\nu$ mass scenarios allowing neutrinos as hot dark matter in the
universe for the small angle MSW solution of the solar neutrino problem. It
probes cosmological models including hot dark matter already now on the level
of future satellite experiments MAP and PLANCK. It further probes many topics
of beyond Standard Model physics at the TeV scale. Future experiments should
give access to the multi-TeV range and complement on many ways the search for
new physics at future colliders like LHC and NLC. For neutrino physics GENIUS
will allow to test almost all neutrino mass scenarios allowed by the present
neutrino oscillation experiments. At the same time GENIUS will cover a wide
range of the parameter space of predictions of SUSY for neutralinos as cold
dark matter. Further it has the potential to be a real-time detector for
low-energy (pp and 7Be) solar neutrinos. A GENIUS Test Facility has just been
funded and will come into operation by end of 2001.
