The twentieth Johns Hopkins Workshop on current problems in particle theory took place in Heidelberg. The topic of the workshop was chosen in view of the phantastic success enjoyed by the standard model of electroweak and strong interactions.Until today, no significant deviations from the predictions of the standard model have been observed. However, precision tests have been dominantly performed in the high-energy domain, where the QCD coupling constant is small enough to allow for a perturbative treatment of the strong interaction. It is therefore very important to consider also the low-energy region for which non-perturbative aspects of QCD come into play.