QCD at low energy: ππ scattering

In this talk I have reviewed recent progress in our understanding of ππ scattering at low energy. Very precise theory predictions have been worked out several years ago and are now being tested at the same level of accuracy by different experiments. In order to do a proper comparison, however, it is essential to evaluate isospin-breaking corrections pertinent to each experiment. I have reviewed recent calculations of these effects and showed that theoretical predictions have so far been confirmed by different experiments at a remarkable precision level.