Polarized structure functions in QCD

Hadron spin physics is now one of the most active fields of physics. Especi ally in the last ten years, great progress has been made both theoretically and experimentally that has considerably improved our knowledge of the spi n structure of nucleons. We review the nucleon's polarized structure functi ons from the viewpoint of factorization theorems and the gauge invariant, n onlocal light-cone operators in QCD. We discuss a systematic treatment of t he polarized structure functions and the corresponding parton distribution functions, which are relevant to inclusive lepto- and hadro-production. We give a detailed analysis of these spin-dependent distribution functions at the twist-2 and twist-3 level, and present Various properties and relations satisfied by the parton distributions, which can be derived directly from QCD. We emphasize unique features of higher twist distributions,and the rol e of the QCD equations of motion to derive their sensitivity to the quark-g luon correlation and their anomalous dimensions for Q(2)-evolution.