  We review the nucleon's polarized structure functions from the viewpoint of
gauge invariant, nonlocal light-cone operators in QCD. We discuss a systematic
treatment of the polarized structure functions and the corresponding parton
distribution functions. We also address a question of what information on the
structure of Nature will be obtained from the future polarized experiments.
  From this point of view, we will discuss the W gamma production at RHIC
polarized experiment.
