  The twenty years since the announcement of the proton spin crisis by the
European Muon Collaboration has seen tremendous progress in our knowledge of
the distribution of spin within the proton. The problem is reviewed, beginning
with the original data and the suggestion that polarized gluons may play a
crucial role in resolving the problem through the U(1) axial anomaly. The
discussion continues to the present day where not only have strong limits have
been placed on the amount of polarized glue in the proton but the experimental
determination of the spin content has become much more precise. It is now clear
that the origin of the discrepancy between experiment and the naive expectation
of the fraction of spin carried by the quarks and anti-quarks in the proton
lies in the non-perturabtive structure of the proton. We explain how the
features expected in a modern, relativistic and chirally symmetric description
of nucleon structure naturally explain the current data.
