  $K_{\ell4}$ are for several reasons an especially interesting decay channel
of $K$ mesons: $K_{\ell4}$ decays allow an accurate measurement of a
combination of $S$-wave $\pi\pi$ scattering lengths, one form factor of the
decay is connected to the chiral anomaly and the decay is the best source for
the determination of some low energy constants of ChPT. We present a dispersive
approach to $K_{\ell4}$ decays, which takes rescattering effects fully into
account. Some fits to NA48/2 and E865 measurements and results of the matching
to ChPT are shown.
