Theory and phenomenology of the QCD vacuum

This paper reviews various approaches used nowadays in order to understand the structure of the ground state of the strong interaction theory, the so-called QCD vacuum. In particular, we discuss semiclassical methods (chapter 2), the lattice formulation (including numerical methods and their recent results, chapter 3) as well as the QCD sum rules for various correlators (chapters 4 and 5). Comments on hadronic structure and reactions are collected in chapter 6. Chapter 7 is devoted to the so-called macroscopic approach in QCD. Perspectives of production and observation of the macroscopic cluster of highly excited matter in high energy collisions are also discussed.