  I will summarize the physics that can be investigated with polarized pp
collisions. It is technically feasible to use the RHIC collider for
accelerating highly polarized protons to a center-of-mass energy of about 400
GeV, with high luminosity. Such collisions can be used to probe the
spin-dependence of hard collisions and of partons in a hadron, including the
gluons. There are interesting twist 3 phenomena that are likely to be
significant in view of the large transverse spin asymmetries at lower energies;
this will have important implications for the spin structure of the proton wave
function. Recent theoretical developments include the possibility of probing
the spin of transversely polarized quarks via asymmetries in the jets they
make.
