  I give a very short introduction to Chiral Perturbation Theory and an
overview of the next-to-next-to-leading order three-flavour calculations done.
I discuss those relevant for an improvement in the accuracy of the measurement
of $V_{us}$ in more detail. One major conclusion is that all needed $p^6$ low
energy constants can be obtained from experiment via the scalar form-factor in
$K_{\ell3}$ decays.
