Physics with Polarized Protons at HERA

The operation of HERA with polarized proton and electron beams will allow to study a wide variety of observables in polarized electron-proton collisions at $\sqrt s=300$ GeV. The physics prospects of this project have been elaborated in detail in a dedicated working group, whose results we summarize in this report. We show that several important and often unique measurements in spin physics could be made at HERA. These include measurements of the polarized structure function $g_1(x,Q^2)$ at low $x$, a direct determination of the polarized gluon distribution $\Delta G(x,Q^2)$ for the region $0.002<x<0.2$ from polarized di-jet rates and hadrons with high $p_t$, polarized quark distributions from weak structure functions and semi-inclusive asymmetries, parton distributions in the polarized photon and information on the helicity structure of possible new physics at large $Q^2$. HERA could therefore make a significant contribution to our understanding of spin effects in high energy collisions and to the spin structure of the nucleon.