The prospects of a physics program which can be addressed with polarized pr
otons in HERA are summarized and compared to those of other existing or pla
nned facilities. It is shown that polarised protons in HERA would provide u
nique access to several spin related observables of the nucleon and the pho
ton in an unexplored kinematical range, if an appropriate level of polariza
tion and luminosity can be achieved.