  The experimental results obtained in the last few years on kaon decays
(K$\to2\pi$ and, above all, Ke4 decays) allow a reliable, model independent
determination of low energy $\pi\pi$ scattering in the S0 wave. Using them and,
eventually, other sets of data, it is possible to give a precise
parametrization of the S0 wave as well as to find the scattering length and
effective range parameter. One can also perform an extrapolation to the pole of
the "$\sigma$ resonance" [$f_0(600)$]. We obtain the results
$$a_0^{(0)}=0.233\pm0.013 M^{-1}_\pi,\quad b_0^{(0)}=0.285\pm0.012 M^{-3}_\pi$$
and, for the $\sigma$ pole, $$M_\sigma=484\pm17 \mev,\quad\gammav_\sigma/2=
255\pm10 {\rm MeV}.$$
