This book is a rigorous, self-contained exposition of the mathematical theory for wave propagation in layered media with arbitrary amounts of intrinsic absorption. The theory, previously unpublished in book form, provides solutions for fundamental wave-propagation problems and corresponding numerical results in the context of any media with a linear response (elastic or anelastic). It provides new insights regarding the physical characteristics for two- and three-dimensional anelastic body and surface waves. The book is an excellent graduate-level textbook. It permits fundamental elastic wave propagation to be taught in the broader context of wave propagation in any media with a linear response. The book is also a valuable reference text. It provides tools for solving problems in seismology, geotechnical engineering, exploration geophysics, solid mechanics, and acoustics. The numerical examples and problem sets facilitate understanding by emphasizing important aspects of both the theory and the numerical results.