  This paper presents the analysis of leptonic and semileptonic kaon decays
data done by the FlaviaNet Kaon Working group. Data include all recent results
by BNL-E865, KLOE, KTeV, ISTRA+, and NA48. Experimental results are critically
reviewed and combined, taking into account theoretical (both analytical and
numerical) constraints on the semileptonic kaon form factors. We report on a
very accurate determination of V_{us} as well as on many other tests of the SM
which can be performed with leptonic and semileptonic K decays.
