  Initial-state and final-state interactions, which are conventionally
neglected in the parton model, have a profound effect in QCD hard-scattering
reactions. These effects, which arise from gluon exchange between the active
and spectator quarks, cause leading-twist single-spin asymmetries, diffractive
deep inelastic scattering, diffractive hard hadronic reactions, and the
breakdown of the Lam-Tung relation in Drell-Yan reactions. Diffractive deep
inelastic scattering also leads to nuclear shadowing and non-universal
antishadowing of nuclear structure functions through multiple scattering
reactions in the nuclear target. Factorization-breaking effects are
particularly important for hard hadron interactions since both initial-state
and final-state interactions appear. Related factorization breaking effects can
also appear in exclusive electroproduction reactions and in deeply virtual
Compton scattering. None of the effects of initial-state and final-state
interactions are incorporated in the light-front wavefunctions of the target
hadron computed in isolation.
