Bound-state properties from field-theory correlators

We discuss the details of calculating hadron properties from the OPE for correlators of quark currents in QCD, which constitutes the basis of the method of QCD sum rules. The main emphasis is laid on gaining control over the systematic uncertainties of the hadron parameters obtained within this method. We start with examples from quantum mechanics, where bound-state properties may be calculated independently in two ways: exactly, by solving the Schroedinger equation, and approximately, by the method of sum rules. Knowing the exact solution allows us to control each step of the sum-rule extraction procedure. On the basis of this analysis, we formulate several improvements of the method of sum rules. We then apply these modifications to the analysis of the decay constants of heavy mesons.