This book serves as a comprehensive guide to the physics of thin-film optical spectra, bridging the gap between fundamental physics courses, such as optics, electrodynamics, quantum mechanics, and solid-state physics, and the highly specialized literature on the spectroscopy, design, and application of optical thin film coatings. It presumes a basic understanding from these courses and builds upon it. Starting from the fundamentals of physics, the book equips the reader with the ability to derive the theory of optical coatings and apply it to significant spectroscopic problems. It encompasses both classical and semiclassical approaches. The topics covered range from classical optical coatings in various spectral regions to more specialized areas such as rugate filters and ultrafast mirrors. The expanded and updated third edition places a stronger emphasis on basic physical modeling aspects and updates the description of nonlinear coating properties. Additionally, it includes an expanded collection of problems with detailed solutions and explanations, enhancing the reader understanding and application of the concepts.