  The proton spin puzzle inspired by the EMC experiment and its present status
are closely examined. Recent experimental progress is reviewed. Various
factorization schemes due to the ambiguity arising from the axial anomaly are
discussed. Some misconceptions in the literature about the $\MS$ factorization
scheme are clarified. It is stressed that the polarized nucleon structure
function $g_1(x)$ is independent of the factorization scheme chosen in defining
the quark spin density. Consequently, the anomalous gluon and sea-quark
interpretations for the deviation of the observed first moment of $g_1^p(x)$
from the Ellis-Jaffe sum rule are equivalent. While it is well known that the
total quark spin in the chiral-invariant (CI) factorization scheme (e.g. the
improved parton model) can be made to be close to the quark model expectation
provided that the gluon spin is positive and large enough, it is much less
known that, contrary to the gauge-invariant scheme (e.g. the MSbar scheme), the
quark orbital angular momentum in the CI scheme deviates even farther from the
relativistic quark model prediction. Recent developments in the NLO analysis of
polarized DIS data, orbital angular momentum and lattice calculations of the
proton spin content are briefly sketched.
