Classical Description of the Interaction of Light with Matter -- The Linear Dielectric Susceptibility General -- The Classical Treatment of Free and Bound Charge Carriers -- Derivations from the Oscillator Model -- The Kramers-Kronig Relations -- Interface Reflection and Interference Phenomena in Thin Film Systems -- Planar Interfaces -- Thick Slabs and Thin Films -- Extended Details: Gradient Index Films and Multilayers -- 9 Special Geometries -- Semiclassical Description of the Interaction of Light with Matter -- Einstein Coefficients -- Semiclassical Treatment of the Dielectric Function -- Solid State Optics -- Basics of Nonlinear Optics -- Some Basic Effects of Nonlinear Optics -- Summary.The book is intended to bridge the gap between fundamental physics courses (such as optics, electrodynamics, quantum mechanics and solid state physics) and highly specialized literature on the spectroscopy, design, and application of optical thin film coatings. Basic knowledge from the above-mentioned courses is therefore presumed. Starting from fundamental physics, the book enables the reader derive the theory of optical coatings and to apply it to practically important spectroscopic problems. Both classical and semiclassical approaches are included. Examples describe the full range of classical optical coatings in various spectral regions as well as highly specialized new topics such as rugate filters and resonant grating waveguide structures.