  We study the discovery potential of future beta decay experiments on searches
for the neutrino mass in the sub-eV range, and, in particular, KATRIN
experiment with sensitivity $m > 0.3$ eV. Effects of neutrino mass and mixing
on the beta decay spectrum in the neutrino schemes which explain the solar and
atmospheric neutrino data are discussed. The schemes which lead to observable
effects contain one or two sets of quasi-degenerate states. Future beta decay
measurements will allow to check the three neutrino scheme with mass
degeneracy, moreover, the possibility appears to measure the CP-violating
Majorana phase. Effects in the four neutrino schemes which can also explain the
LSND data are strongly restricted by the results of Bugey and CHOOZ oscillation
experiments: Apart from bending of the spectrum and the shift of the end point
one expects appearance of small kink of ($ < 2% $) size or suppressed tail
after bending of the spectrum with rate below 2 % of the expected rate for zero
neutrino mass. We consider possible implications of future beta decay
experiments for the neutrino mass spectrum, the determination of the absolute
scale of neutrino mass and for establishing the nature of neutrinos. We show
that beta decay measurements in combination with data from the oscillation and
double beta decay experiments will allow to establish the structure of the
scheme (hierarchical or non-hierarchical), the type of the hierarchy or
ordering of states (normal or inverted) and to measure the relative
CP-violating phase in the solar pair of states.
