SNAPSHOTS OF HADRONS - OR THE STORY OF HOW THE VACUUM MEDIUM DETERMINES THE PROPERTIES OF THE CLASSICAL MESONS WHICH ARE PRODUCED, LIVE ANDDIE IN THE QCD VACUUM

I discuss the modern development of the analytic methods in the theory of hadrons. These methods are based on the operator product expansion and ascend to the SVZ sum rules elaborated 20 years ago. They allow o ne to relate a large variety of properties of the low-lying hadronic s tates and important regularities established in the hadronic world to a few vacuum condensates parametrizing the vacuum of quantum chromodyn amics. A significant progress took place in deeper understanding of bo th, the conceptual foundations of the method, and its practical implem entation and limitations. I review, from the modern standpoint, novel ideas of the QCD vacuum, various aspects of the operator product expan sion, new applications of the SVZ sum rules, and their role among othe r competing approaches in the theory of hadrons.