  The low energy part of the reactor neutrino spectra has not been
experimentally measured. Its uncertainties limit the sensitivities in certain
reactor neutrino experiments. The origin of these uncertainties are discussed,
and the effects on measurements of neutrino interactions with electrons and
nuclei are studied. Comparisons are made with existing results. In particular,
the discrepancies between previous measurements with Standard Model
expectations can be explained by an under-estimation of the low energy reactor
neutrino spectra. To optimize the experimental sensitivities, measurements for
$\nuebar$-e cross-sections should focus on events with large ($>$1.5 MeV)
recoil energy while those for neutrino magnetic moment searches should be based
on events $<$100 keV. The merits and attainable accuracies for
neutrino-electron scattering experiments using artificial neutrino sources are
discussed.
