As in former editions of this book, I have expanded the present one to contain new sections on subjects beyond those found in the previous editions, thereby filling some of the existing gaps. Thus, we now have a section on heavy quark effective theories, one of the more powerful methods devised to take advantage of the existence a large scale (the mass of the heavy quark) for observables involving at least a heavy quark; a new section on lattice QCD, a method of calculation that is already giving reasonably reliable results for a number of observables; and several sections with a detailed discussion of chiral perturbation theory. There is little doubt that, at least at the one loop level, chiral perturbation theory has led to an important insight into pion physics in a region where methods derived from ordinary perturbation theory are not at applicable, which justifies this substantial addition. Besides this, I have corrected errors in the previous edition and improved, here and there, the material on several among the various topics; in particular, including more information on experiment, and on comparison of theory with experiment. The values of the basic QCD parameters have also been completely updated.