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.