  The proton spin problem triggered by the EMC experiment and its present
status are closely examined. Recent experimental and theoretical progresses and
their implications are reviewed. It is pointed out that the sign of the
sea-quark polarization generated perturbatively by hard gluons via the anomaly
mechanism is predictable: It is negative if the gluon spin component is
positive. We stress that the polarized nucleon structure function $g_1(x)$ is
independent of the $k_\perp$-factorization scheme chosen in defining the quark
spin density and the hard photon-gluon scattering cross section. Consequently,
the anomalous gluon and sea-quark interpretations for $\Gamma_1$, the first
moment of $g_1(x)$, are equivalent. It is the axial anomaly that accounts for
the observed suppression of $\Gamma_1^p$.
