Masses of the Light Quarks
  The magnitude of $m_u,\,m_d$ and $m_s$ is discussed on the basis of Chiral Perturbation Theory. In particular, the claim that $m_u=0$ leads to a coherent picture for the low energy structure of QCD is examined in detail. It is pointed out that this picture leads to violent flavour asymmetries in the matrix elements of the scalar and pseudoscalar operators, which are in conflict with the hypothesis that the light quark masses may be treated as perturbations. [Talk delivered at the 2nd IFT Workshop, "Yukawa Couplings and the Origins of Mass", Gainesville, Florida, Feb. 1994, to be published by International Press.] 
H. Leutwyler