  The form factors of the light pseudoscalar mesons are investigated in a
dispersive formalism based on hadronic unitarity, analyticity and the OPE
expansion of the QCD Green functions. We propose generalizations of the
original mathematical techniques, suitable for including additional low energy
information provided by experiment or Chiral Perturbation Theory (CHPT). The
simultaneous treatment of the electroweak form factors of the $\pi$ and $K$
mesons allows us to test the consistency with QCD of a low energy CHPT theorem.
By applying the formalism to the pion electromagnetic form factor, we derive
quite strong constraints on the higher Taylor coefficients at zero momentum,
using information about the phase and the modulus of the form factor along a
part of the unitarity cut.
