  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.
