  The 1/N_c expansion provides a theoretical method for analyzing the
spin-flavor symmetry properties of baryons in QCD that is quantitative,
systematic and predictive. An exact spin-flavor symmetry exists for large-N_c
baryons, whereas for QCD baryons, the spin-flavor symmetry is approximate and
is broken by corrections proportional to the symmetry-breaking parameter 1/N_c
= 1/3. The 1/N_c expansion predicts a hierarchy of spin and flavor symmetry
relations for QCD baryons that is observed in nature. It provides a
quantitative understanding of why some SU(3) flavor symmetry relations in the
baryon sector, such as the Gell-Mann--Okubo mass formula, are satisfied to a
greater precision than expected from flavor symmetry-breaking suppression
factors alone.
