  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 practically
excludes degenerate $\nu$ mass scenarios allowing neutrinos as hot dark matter
in the universe for the smallangle 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 SM 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 some of them
(GENIUS) will allow to test almost all neutrino mass scenarios allowed by the
present neutrino oscillation experiments.
