  The physics scope of polarized nucleon-nucleon collisions originating from an
internal polarized target in the HERA proton beam is summarized. Based on 240
pb^{-1} integrated luminosity at 40 GeV c.m. energy, statistical sensitivities
are given over a wide (x_F, p_T)-range for a variety of inclusive and exclusive
final states. By measuring single spin asymmetries unique information can be
obtained on higher twist contributions and their p_T-dependence. From double
spin asymmetries in both photon and J/psi production it appears possible to
measure the polarized gluon distribution in the range 0.1 < x_{gluon} < 0.4
with a good statistical accuracy. Drell-Yan pair production asymmetries in
doubly longitudinal mode, measurable in the mass range 3-10 GeV, allow
discrimination between different parametrizations of the polarized light sea
quark distributions. In doubly transverse mode access to the quark transversity
distribution might become possible. Statistically significant results can be
expected in the elastic channel.
