Spin Physics: past, present and future

Nucleons are known to be building blocks of matter and also to be bound states of quarks and gluons - the partons, whose dynamics is best described by Quantum Chromodynamics (QCD). Perturbative QCD has been successful in interpreting and predicting high energy scattering processes, in extracting the information on short-distance partonic dynamics, and in measuring the parton momentum distribution inside the nucleon. I will demonstrate how to use perrurbative QCD and high energy scattering processes to understand the spin structure of the nucleon: where does the spin of the nucleon come from? First we will review the current status of quark and gluon spin contributions, which could be extracted from longitudinal spin phenomena. Second we will discuss how to probe the parton"s transverse motion, which is related to orbital motion and could be extracted from transverse spin phenomena. Under the quest for spin structure, we will find that the spin has become a powerful tool to study QCD dynamics, factoration, and gauge invariance, which makes spin physics one of the most active fields in nuclear physics