  We present a calculation of the leptonic decay-constant ratio $f_K/f_\pi$ in
2+1 flavor QCD. Our data set includes five lattice spacings and pion masses
reaching down below the physical one. Special emphasis is placed on a careful
study of all systematic uncertainties, especially the continuum extrapolation.
Our result is perfectly compatible with the first-row unitarity constraint of
the Standard Model.
