  Besides the opportunity for discovering new neutrino physics, solar neutrino
measurements provide a sensitive probe of the solar interior, and thus a
rigorous test of solar model predictions. We present model independent
determinations of the neutrino spectrum by using relevant flux components as
free parameters subject only to the luminosity constraint. (1) Without the
Mikheyev-Smirnov-Wolfenstein (MSW) effect, the best fit for the combined data
is poor. Furthermore, the data indicate a severe suppression of the $^7$Be flux
relative to the $^8$B, contradicting both standard and nonstandard solar models
in general; the $pp$ flux takes its maximum value allowed by the luminosity
constraint. This pathology consistently appears even if we ignore any one of
the three data. (2) In the presence of the two-flavor MSW effect, the current
constraint on the initial $^8$B flux is weak, but consistent with the SSM and
sufficient to exclude nonstandard models with small $^8$B fluxes. No meaningful
constraint is obtained for the other fluxes. In the future, even allowing MSW,
the $^8$B and $^7$Be fluxes can be determined at the $\pm$(15 -- 20)\% level,
making competing solar models distinguishable. We emphasize that the neutral
current sensitivity for $^7$Be neutrinos in BOREXINO, HELLAZ, and HERON is
essential for determining the initial fluxes. The constraints on the MSW
parameters in the model independent analysis are also discussed.
