Chiral dynamics with strange quarks

In the first part of the talk, I review what we know (or rather do not know) about the structure of the QCD vacuum in the presence of strange quarks. Chiral perturbation theory allows to study reactions of pions and kaons and to further sharpen our understanding of symmetry violation in QCD. I review recent progress on the description of pion-kaon scattering, in particular concerning isospin violation and the extraction of threshold and resonance parameters from Roy-Steiner equations. In the third part, it is shown how a unitary extension of chiral perturbation theory leads to novel insight into the structure of the Λ(1405)