2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/42441I give an overview of the calculations done in three-flavour Chiral perturbation theory at next-to-next-to-leading order with an emphasis on those relevant for an improvement in the accuracy of the measurement of $V_{us}$. It is pointed out that all needed low energy constants can be obtained from experiment via the scalar form-factor in $K_{\ell3}$ decays4 pages, talk given at 38th Rencontres de Moriond on QCD and High-Energy Hadronic Interactions, Les Arcs, Savoie, France, 22-29 Mar 2003High Energy Physics - Phenomenologytext