  Lecture Notes, Summer School on Masses of Fundamental Particles, Cargese,
1996. In the first part, some qualitative aspects the mass pattern are
discussed, concerning the breaking of isospin and eightfold way symmetries. The
second part deals with the chiral perturbation theory results for the masses of
the pseudoscalar octet, which lead to very stringent constraints for the
relative size of the light quark masses.
