  The Operator Product Expansion (OPE) of current correlators at short
distances beyond perturbation theory in QCD, together with Cauchy's theorem in
the complex energy plane, are the pillars of the method of QCD sum rules. This
technique provides an analytic tool to relate QCD with hadronic physics at low
and intermediate energies. It has been in use for over thirty years to
determine hadronic parameters, form factors, and QCD parameters such as the
quark masses, and the running strong coupling at the scale of the
$\tau$-lepton. QCD sum rules provide a powerful complement to numerical
simulations of QCD on the lattice. In this talk a short review of the method is
presented for non experts, followed by three examples of recent applications.
