  $K_{\ell4}$ decays offer several reasons of interest: they allow an accurate
measurement of $\pi\pi$-scattering lengths; they provide the best source for
the determination of some low-energy constants of ChPT; one form factor is
directly related to the chiral anomaly, which can be measured here. We present
a dispersive treatment of $K_{\ell4}$ decays that provides a resummation of
$\pi\pi$- and $K\pi$-rescattering effects. The free parameters of the
dispersion relation are fitted to the data of the high-statistics experiments
E865 and NA48/2. The matching to ChPT at NLO and NNLO enables us to determine
the LECs $L_1^r$, $L_2^r$ and $L_3^r$. With recently published data from
NA48/2, the LEC $L_9^r$ can be determined as well. In contrast to a pure chiral
treatment, the dispersion relation describes the observed curvature of one of
the form factors, which we understand as a rescattering effect beyond NNLO.
