QCD tests with kaons

Final results from an analysis of about 400 $K^{\pm} \rightarrow \pi^{\pm}\gamma\gamma$ rare decay candidates collected by the NA48/2 and NA62 experiments at CERN are presented. The results include a model-independent decay rate measurement and fits to Chiral Perturbation Theory (ChPT) description. The data support the ChPT prediction for a cusp in the di-photon invariant mass spectrum at the two pion threshold. The NA48/2 Collaboration at CERN has accumulated unprecedented statistics of rare kaon decays in the $K_{e4}$ modes $K^{\pm}_{e4} \rightarrow \pi^{+}\pi^{−}e\nu$ and $K^{00}_{e4} \rightarrow \pi^{0}\pi^{0}e\nu$ with one percent background contamination. The detailed study of the form factors brings new inputs to low energy QCD description and crucial tests of predictions from ChPT and lattice QCD calculations.