The confinement mechanism of the quarks in QCD is one of the most challenging and open problems in physics. Confinement is a nonperturbative phenomenon, and a definite way to handle it has not yet been found in field theory. There are lattice calculations that can produce the low-lying states of the spectrum and 'measure' many important physical quantities, but nevertheless the development of analytical techniques is of extreme importance for understanding the physics involved in confinement. In this respect it is important to test the results obtained directly from the theory (Bethe-Salpeter kernel, effective Hamiltonians, quark potential, etc.) on the spectrum, form factors and decays of bound states of quarks and gluons, and to relate them to the results of lattice theory.In this book, the question of the confinement mechanism is addressed; explanations in terms of monopoles, instantons and dyons are reviewed and the connection with duality is discussed.