  Several important and unique measurements, within the standard model and of
possible physics beyond it, could be made with {\em polarized HERA} in which
both the proton and the electron beams are polarized. With a $\sim$820 GeV
proton beam and a $\sim$27.6 GeV electron beam, the polarized HERA will enable
$\vec{e}-\vec{p}$ collisions with $\sqrt{s} \sim 300$ GeV and access spin
variables in the kinematic range, $10^{-5} \le x_{\rm Bj} \le 0.6$ and $0 \le
Q^{2} \le 10^{5}$, using the H1 and ZEUS detectors at DESY. This will be an
increase of two orders of magnitude in both $x$ and $Q^{2}$ range compared to
the presently explored range from fixed target experiments at CERN, SLAC and
DESY. No other approved or planned spin experiment or accelerator facility will
access the low $x$ and high $Q^{2}$ regions possible with HERA. Measurements
performed with the polarized HERA collider will include the polarized structure
function $g_{1}(x,Q^{2})$ at very low $x$, the polarized gluon distribution
$\Delta G(x,Q^{2})$ from pQCD analysis of $g_{1}$, and from the production of
di-jet events and high-p$_{T}$ hadrons in photon gluon fusion process and in
photoproduction, the weak structure functions, the valence quark distribution
functions from semi-inclusive asymmetries, parton distributions inside
polarized photon, and information on helicity structure of possible new physics
beyond the standard model. With such a rich and broad physics program possible
for HERA,not polarizing the proton beam would be a great opportunity lost.
