An overview is given of weak interaction physics of the light flavours.
It starts with the definition of the CKM matrix and the measurement of
its components in the light-flavour sector via semi-leptonic decays. The
main part of the lectures is devoted to non-leptonic decays with a main
emphasis on analytical calculations of $K\to\pi\pi$ and $K^0\leftrightarrow
\kob$ mixing. It finishes with an overview of Chiral Perturbation Theory
in $K\to3\pi$ and of rare Kaon decays.