  I draw attention to a recent breakthrough in the field of low energy pion
physics: the consequences of the hidden symmetry of the QCD Hamiltonian have
successfully been incorporated in the general dispersive framework for the
pi-pi scattering amplitude, which is due to Roy. The meagre experimental
information about the imaginary parts at and above 0.8 GeV suffices to
unambiguously and accurately pin down the scattering amplitude at lower
energies. The recent Brookhaven data on the reaction K -> pi pi e nu provide a
significant test of the theory. They imply that the Gell-Mann-Oakes-Renner
relation is approximately valid -- the bulk of the pion mass indeed originates
in the quark condensate.
