The phenomenological picture of the Pomeron has considerably evolved in recent years, through theoretical work to establish connections with QCD concepts in terms of multi-gluon-exchange and through experimental studies of some hard features of diffraction. The resulting structure of the Pomeron, originally considered as a simple Regge trajectory, mainly imaginary and helicity-conserving, thus implying vanishing spin effects in elastic scattering with increasing energy, is now more complex and should be probed at the level of helicity amplitudes. For hard scattering at high momentum- transfer, QCD helicity conservation at costituent level implies significant spin correlations in the scattering of polarized protons.

Several features of existing elastic scattering polarization data at high energy have stimulated discussions on possible spin-flip contributions that do not necessarily decrease with CM-energy or with |t|; characteristic patterns of experimental spin asymmetries for pp elastic scattering are described and discussed in comparison with current models. The interest and the experimental possibilities to extend these types of measurements to higher energies with polarized protons colliding in RHIC are emphasized.