  I discuss the recent attempts to build an effective chiral Lagrangian
incorporating massive resonance states. A useful approximation scheme to
organize the resonance Lagrangian is provided by the large-Nc limit of QCD.
Integrating out the resonance fields, one recovers the usual chiral
perturbation theory Lagrangian with explicit values for the low-energy
constants, parameterized in terms of resonance masses and couplings. The
resonance chiral theory generates Green functions that interpolate between QCD
and chiral perturbation theory. Analyzing these Green functions, both for large
and small momenta, one gets QCD constraints on the resonance couplings and,
therefore, information on the low-energy constants governing the Goldstone
interactions.
