Three experts in the field of thin-film optics present a detailed and self-contained theoretical study of planar multilayers and how they can be effectively exploited in both traditional and modern applications. Starting with a discussion of the relevant electromagnetic optics, the fundamental optical properties of multilayers are introduced using an electromagnetic approach based on a direct solving of Maxwell's equations by Fourier transforms. This powerful approach is illustrated through the comprehensive description of two of the most important phenomena in multilayers, i.e. giant field enhancement in dielectric stacks and light scattering from thin-film optical filters. The same approach is extended to the description of the operation of planar microcavities and the balance of energy between radiated and trapped light. This book will be valuable to researchers, engineers and graduate students with interests in nanophotonics, optical telecommunications, observational astronomy and gravitational wave detection.