The book examines in detail the theory of interference phenomena in single-layer and multi-layer thin coatings and elucidates the optical properties of coatings of various types (metal, dielectric, monomolecular). The foundations of algebraic optics have been developed and the matrix equation of radiation transfer has been formulated. The invention describes use of coatings for illuminating optics and making selective mirrors and polarisers. Particular attention is paid to the use of layered interferometers to solve a number of practical problems (interference filters, microrelief study of surfaces, determination of optical constants, etc.)