  Nucleon spin structure has been an active and exciting subject of interest
for the last three decades. Recent precision spin-structure data from Jefferson
Lab have significantly advanced our knowledge of nucleon structure in the
valence quark (high-x) region and improved our understanding of higher-twist
effects, spin sum rules and quark-hadron duality. First, results of spin sum
rules and polarizabilities in the low to intermediate Q^2 region are presented.
Comparison with theoretical calculations are discussed. Surprising
disagreements of Chiral Perturbation Theory calculations with experimental
results on the generalized spin polarizability, \delta_{LT}, were found. Then,
precision measurements of the spin asymmetry in the high-x region are
presented. They provide crucial input for global fits to world data to extract
polarized parton distribution functions. The up and down quark spin
distributions in the nucleon were extracted. The results for \Delta d/d
disagree with the leading-order pQCD prediction assuming hadron helicity
conservation. Results of precision measurements of the g_2 structure function
to study higher-twist effects are presented. The data indicate a significant
higher-twist (twist-3 or higher) effect. The second moment of the spin
structure functions and the twist-3 matrix element d_2 results were extracted.
The high Q^2 result was compared with a Lattice QCD calculation. Results on the
resonance spin-structure functions in the intermediate Q^2 range are presented,
which, in combination with DIS data, enable a detailed study of quark-hadron
duality in spin-structure functions. Finally, an experiment to study neutron
transversity and transverse spin asymmetries is discussed. A future plan with
the 12 GeV energy upgrade at JLab is briefly outlined.
