The subject of the book is CVD thin film optical coatings for application in photothermal solar energy conversion. The specific character of the materials research in the area of thermal solar energy conversion is the necessity of large areas for interception of the solar flux. The interceptive surfaces meeting the solar flux are mainly thin film (for cost effective purposes) coatings and the price of their production should be very low. Besides they must withstand high temperatures reaching in some applications 500 degrees C, and the film should be stable for a long time (15-20 years). The possible diffusion processes lead to structure changes in thin films and deterioration of optical properties. These coatings differ from the ones for thin film optics, where the applications are basically at room temperature. These coatings differ also from the microelectronics materials, where miniaturisation lowered the price.