2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/45800I summarize the results of the complete one--loop chiral perturbation theory calculation performed recently. It is shown that it allows to accurately pin down the isospin two, S--wave $ππ$ scattering length $a_0^2$. On the other hand, interesting resonance physics makes a precise determination of $a_0^0$ very difficult.4 pp, LaTeX, Invited talk, BARYONS' 95, Santa Fe, October 1995, to appear in the ProceedingsHigh Energy Physics - PhenomenologyNuclear Theorytext