There is no compelling laboratory evidence for non-zero neutrino mass. The direct limits from kinematic searches for the masses yield the upper limits [1]
On the other hand, most extensions of the standard predict non-zero masses
at some level [2]. Unified theories and other extended gauge
groups with a large mass scale 
often predict a seesaw-type mass
, where v = 246 GeV is the weak scale. Many
other models with Higgs triplets or loops involving new Higgs particles
also generate neutrino mass at some level. There are also several hints
for non-zero mass from astrophysics and cosmology. All four solar neutrino
experiments, Homestake [3], Kamiokande [4], SAGE
[5], and GALLEX [6], observe a deficit of neutrinos
compared to the standard solar model expectation [7], suggesting
that there is either nonstandard astrophysics or new neutrino properties.
However, the relative rates observed by Homestake and Kamiokande yield a
simple measure of the distortion of the spectral shape, suggesting that
most of the suppression is in the middle of the spectrum, i.e., of
the
line and probably the low energy part of the 
spectrum, with less
suppression at lower and higher energies [8]--[14].
This cannot
be accounted for by any known astrophysical or nuclear physics explanation
and strongly suggests new neutrino properties. More recently, the two
gallium experiments, SAGE and GALLEX, have accumulated reasonably good
statistics. Their rates are low compared to any reasonable solar model,
standard or nonstandard, again suggesting the need for new neutrino
properties. One can infer the suppression of the 
line from any two
types of experiment, gallium/Kamiokande, gallium/Homestake, or
Homestake/Kamiokande.
Amongst the new neutrino properties the Mikheyev-Smirnov-Wolfenstein (MSW)
[15] mechanism of matter-enhanced neutrino oscillations explains the
data very well, and does suppress the middle of the spectrum for some
regions of neutrino mass and mixings [16] -- [25]. Although
it remains to be verified by future experiments, the MSW mechanism
is quite promising
and would be extremely exciting for particle physics. Solar neutrinos are
also useful for
astronomy. Either with or without the MSW effect one will be able to use
them to probe the different components of the neutrino
spectrum [10] and therefore to do astronomy on the solar core.
Even with present data, if one accepts the 2-flavor MSW interpretation
the neutrino parameters and the temperature
of the core of the sun
can be simultaneously determined
[10], yielding 
, where 
K is the prediction of the standard
solar model [26,27]. Similarly, one can use the
present data, again assuming MSW, to simultaneously determine the flux of
neutrinos. One finds [10] 
for the flux relative to the standard solar model prediction.
Another hint of neutrino mass is the anomalous ratio
suggested by underground searches for neutrinos produced by
interactions of cosmic rays in the atmosphere [28].
Finally, the combination of COBE data [29] and the distribution of
galaxies on large and small scales is hard to understand on the basis of
simple cold dark matter. One possibility is that in addition to cold dark
matter there is a small admixture [30] of hot dark matter,
presumably due to a massive 
neutrino with a mass in the range
[31]. There are, however,
alternative explanations, such as a cosmological constant, topological
defects, or a tilted initial spectrum.
If one accepts some or all of these hints for neutrino mass, the typical
range 
few 
is consistent with the general seesaw-type
scale [32,33] with new physics around 
. This is a typical value that might be expected for the
heavy neutrino mass scale in supersymmetric grand unification [34].
