The perturbative QCD factorization approach in high energy nuclear collisions

I discuss the systematic modifications to the perturbative QCD factorization approach in high energy l+A, p+A and A+A reactions. These include transverse momentum diffusion manifest in the Cronin effect and a small increase in the dijet acoplanarity; nuclear size enhanced power corrections that lead to shadowing in deeply inelastic scattering and suppression of single and double inclusive hadron production at forward rapidity at RHIC but disappear as a function of the transverse momentum; inelastic attenuation of the jet cross sections or jet quenching that persists to much higher pT.