Through the 'magic' of averages it is shown that Maxwell's equations can be used in a simple way to describe filamentory composites as well as pure bulk superconductors. This book, like the first edition, is written for both physicists and engineers. It starts with a rigorous and partialy 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 new chapters on high-Tc materials have been updated, including a change of units to SI and the addition of some historical perspective.