  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
Lambda(1405).
