The book represents a thorough study of MoO3 as an alternative to the well known member of chromogenic materials which have a photochromic phenomenon. Based on that, it has wide application in optoelectronic solar energy modulation and saving energy. These solar control coatings, however, pose a problem in varying climates such as in northern and central Europe. For these cases, materials that have altering properties owing to external surroundings could be the solution. These 'chromatic' materials include several categories, such as photochromic glasses and polymers, thermochromic metal oxides and electrochromic materials.The book is concerned with photochromic materials which has high response to solar radiation. The best form for coating the window glass for building and vehicles is the thin film. Molybdenum oxide thin film is selected to be studied since it could be prepared with the available facilities. Moreover the published mechanisms for the photochromic phenomenon still under debate.