Quantum Chromodynamics (QCD) is a well-tested theory. It describes the strong interaction between quarks through the exchange of gluons. QCD is believed to be well understood at short distances where asymptotic freedom makes a perturbative treatment possible. But QCD is no longer well understood as soon as the basic quark-gluon coupling is not weak anymore; this occurs at distance scales of the order of 10-15 m, i.e. the typical size of a hadron. Then non-perturbative phenomena seem to lead to the formation of hadrons, hadronic mass-generation, confinement and chiral symmetry breaking. These are phenomena which are major puzzles of hadronic physics. To approach a solution presents a profound intellectual challenge for both experiment and theory.