  A new tool which combines chiral perturbation theory and unitarity in coupled
channels is applied with success to the study of the meson-meson interaction,
extending the theoretical predictions up to sqrt{s} = 1.2 GeV, hence improving
considerably the convergence radius of conventional chiral perturbation theory.
The method is applied to obtain the meson-meson phase shifts and inelasticities
as well as the isoscalar pion-pion scattering amplitude inside a nuclear
medium.
