The present monograph represents itself as a tutorial to the field of optical properties of thin solid films. It is neither a handbook for the thin film prac- tioner, nor an introduction to interference coatings design, nor a review on the latest developments in the field. Instead, it is a textbook which shall bridge the gap between ground level knowledge on optics, electrodynamics, qu- tum mechanics, and solid state physics on one hand, and the more specialized level of knowledge presumed in typical thin film optical research papers on the other hand. In writing this preface, I feel it makes sense to comment on three points, which all seem to me equally important. They arise from the following (- tually interconnected) three questions: 1. Who can benefit from reading this book? 2. What is the origin of the particular material selection in this book? 3. Who encouraged and supported me in writing this book? Let me start with the first question, the intended readership of this book. It should be of use for anybody, who is involved into the analysis of - tical spectra of a thin film sample, no matter whether the sample has been prepared for optical or other applications. Thin—lm spectroscopy may be r- evant in semiconductor physics, solar cell development, physical chemistry, optoelectronics, and optical coatings development, to give just a few ex- ples. The book supplies the reader with the necessary theoretical apparatus for understanding and modelling the features of the recorded transmission and reffection spectra.