Johan Bijnens Chiral perturbation theory in the meson sector Abstract The present status of Chiral Perturbation Theory in the meson sector is discussed concentrating on recent developments. This write-up contains short discussions on a listing a few historical papers, the principles behind ChPT, two-flavour ChPT including some comments about the pion polarizability, three-flavour ChPT with a discussion of the recently found relations as tests of ChPT and preliminary results of a new fit of the NLO low-energy-constants (LECs). It discusses somewhat deeper $\eta\to3\pi$ and the arguments for the existence of a hard pion ChPT'' and its application to $K\to2\pi$. LU TP 09-25