Johan Bijnens Hard Pion Chiral Perturbation Theory: What is it and is it relevant for $η^\prime$ decays? Abstract In this talk I give a short introduction to hard pion Chiral Perturbation Theory and an overview of the available applications $K\to\pi\pi$, $B,D\to D,\pi,K,\eta$ semileptonic decays and $\chi_{c0,2}\to\pi\pi,KK$. It is pointed out that the reults for the semileptonic decays obey the LEET relation between $f_+$ and $f_-$. LU TP 11-39