Elliot Leader (Imperial College)
Spin-dependent measurements, utilizing polarized beams and/or targets, have an uncanny ability to expose weaknesses and flaws in theories which are otherwise successful. We briefly recall a couple of classic cases. We then look at two contemporary cases where QCD, or at least our usual formulation of QCD, is in some difficulty, and finally at an alarming new example where our fundamental undersatnding of a very basic reaction seems to be in trouble.