Introduction to QCD sum rules

Even if QCD is the correct theory of strong interactions, we are still a long way off from being able to actually calculate hadronic properties, such as the masses and couplings of observed particles, from the fundamental lagrangian of the theory. Since the determination of the spectrum of QCD is a problem which presumably will never be solved exactly one must of course introduce either some approximation scheme or some model (usually both). Several such schemes or models have been developed over the years. They are Monte Carlo simulations on a lattice, bag models, potential models, QCD sum rules. Since all these approaches have claimed some successes in hadron spectroscopy it is perhaps useful to discuss briefly what can be learned from them as well as what they can teach us about QCD itself