Through the 'magic' of averages it is shown that Maxwell's equations can be used in a simple way to describe filamentary composites as well as pure bulk superconductors. The second edition of AC Loss and Macroscopic Theory of Superconductors, like the first edition, is written for both physicists and engineers. It starts with a rigorous and partially new derivation of Maxwell's equations as they apply to superconductors, and the remainder of the book is devoted to loss approximations for a wide range of applications. The chapters on high-Tc materials were updated and include a change to SI units and the addition of some historical perspective.