The main purpose of the Flavianet kaon working group is to perform
precision tests of the Standard Model and to determine fundamental
couplings, such as Vus, as well as QCD observables (ratio of the decay
constants FK/Fpi, Kl3 form factors, pi-pi scattering lengths..) as
precisely as possible. These latters play a crucial role in our
understanding of the meson dynamics at low energy.       In this talk, I
will review some theoretical aspects of the activities of this working
group based on recent experimental results from NA48, KLOE, KTeV and
ISTRA on leptonic, semileptonic and non-leptonic K decays. I will also
briefly discuss the theoretical interests of the upcoming rare kaon
decay experiments.