  Based on a series of lattice calculations we determine the ratio FK/Fpi in
QCD. With experimental data from kaon decay and nuclear double beta decay, we
obtain a precise determination of |Vus|. Our simulation includes 2+1 flavours
of sea quarks, with three lattice spacings, large volumes and a simulated pion
mass reaching down to about 190 MeV for a full control over the systematic
uncertainties.
