  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 a precision measurement of the
neutron spin asymmetry, $A_1^n$, in the high-$x$ region are discussed. The new
data shows that $A_1^n$ becomes positive at high $x$. They provide crucial
input for the 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. Then, results of a
precision measurement of the $g_2^n$ structure function to study higher-twist
effects are presented. The data show a clear deviation from the lead-twist
contribution. The second moment of the spin structure functions and the twist-3
matrix element $d_2^n$ results were extracted at a high $Q2$ of 5 GeV$^2$.
Results for $d_2^n$ at low-to-intermediate $Q2$ from 0.1 to 0.9 GeV$^2$ were
also extracted from the JLab data. In the same $Q2$ range, the $Q2$ dependence
of the moments of the nucleon spin structure functions was measured, providing
a unique bridge linking the quark-gluon picture of the nucleon and the coherent
hadronic picture. Sum rules and generalized forward spin polarizabilities were
extracted. Finally, preliminary results were presented on the resonance spin
structure functions in the $Q2$ range from 1 to 4 GeV$^2$ to study the
quark-hadron duality.
