Quantum chromodynamics is the fundamental theory of strong interactions. It is a physical theory describing Nature. In this book, however, the reader will not find a discussion on the phenomenological aspect of QCD. What he will find is a profound discussion on the theoretical foundations of QCD: What is gauge symmetry on the classical and on the quantum level? What is the path integral in field theory? What is the QCD vacuum state? What is the effective low energy dynamics of QCD? How to calculate Feynman graphs? What QCD effects are of a genuinely non-perturbative nature? And so on. The book is based on a course of lectures given by the author for graduate students at ITEP, Moscow. Much extra material (such as a rigorous derivation for the reduction formula or for non-Abelian Ward identities) has been added, however.