Theoretical Aspects of the Pion-Pion Interaction
  We give a brief review of the theoretical description of low energy pion-pion scattering by the combined use of Chiral Perturbation Theory and Roy equations, an update of the Regge parametrization of $\pi\pi$ cross sections at high energies, and a short discussion of the scalar radius of the pion. 
Irinel Caprini, Gilberto Colangelo and Heinrich Leutwyler