Twenty years have passed since the European Muon Collaboration announced that almost none of the spin of the proton was carried by its quarks -- the famous "proton spin crisis". We review the original work and initial theoretical ideas proposed to explain this finding before turning attention to the enormous progress that has been made over the two decades since that announcement. We show that within the present, much reduced experimental errors the spin problem has been resolved. The explanation is compared with both lattice QCD and recent data involving deeply virtual Compton scattering and found to be consistent. The response to this challenge has taught us a great deal about the non-perturbative structure of the nucleon.

ANL Physics Division Colloquium Schedule