  A simple matching procedure is proposed to extract constraints on effective
meson theories. In this way, a QCD prediction for the pion decay constant is
found, F(pi)=2 m(pi)/pi, i.e. approximately 90MeV. The same procedure also
determines other mesonic observables, like the decay width of the sigma meson
to two photons. Finally, some information which can be gained about the
hadronic light-by-light contributions to the muon anomalous moment are briefly
commented.
