A wide range of key topics is covered. In Volume 1, the first two chapters are devoted to quantum chromodynamics as the theory of hadrons. The author gives an in-depth discussion of a variety of powerful methods based on Wilson's operator product expansion. Chapter 3 (written together with V Novikov, A Vainshtein, and V Zakharov) is the most systematic and pedagogical presentation of instantons in the gauge theories one can find in the literature. Chapter 4 introduces supersymmetry. Chapter 5, concluding this volume, reviews the fascinating dynamics of supersymmetric gauge theories in the strong coupling regime. Chapter 6, which opens Volume 2, is a culmination of the supersymmetric theme. It gives a state-of-the-art description of the breakthrough developments in supersymmetric gauge theories. It has been written specifically for this book by A Vainshtein and the author. Chapter 7 is designed as a primer of two-dimensional conformal field theory, which constitutes the basis of modern string theory. Chapter 8, the last, presents remarkable new findings in quantum mechanics. Every chapter contains exercises and a list of recommended literature.