Chiral Perturbation Theory and the Quenched Approximation of QCD
  The quenched approximation for QCD is, at present and in the foreseeable future, unavoidable in lattice calculations with realistic choices of the lattice spacing, volume and quark masses. In these lectures, I review the analytic study of the effects of quenching based on chiral perturbation theory. Quenched chiral perturbation theory leads to quantitative insight into the difference between quenched and unquenched QCD, and reveals clearly diseases which plague quenched QCD. A short review of the ideas underlying chiral perturbation theory is included. 
Maarten F.L. Golterman