Flavour physics represents one of the most interesting and, at the same time,
less understood sector of the Standard Theory. On the one hand, the peculiar
pattern of quark and lepton masses, and their mixing angles, may be the clue to
some new dynamics occurring at high-energy scales. On the other hand, the
strong suppression of flavour-changing neutral-current processes, predicted by
the Standard Theory and confirmed by experiments, represents a serious
challenge to extend the Theory. This article reviews both these aspects of
flavour physics from a theoretical perspective.Comment: Contribution to "The Standard Theory up to the Higgs discovery - 60
years of CERN", Ed. by L. Maiani and G. Rolandi (v2: minor typos corrected
