Photochromic polymer systems are of two main types: those which are merely solid solutions of photochromes in polymeric matrices and those custom-designed polymers which inherently exhibit photochromism. This book provides a concise review of developments in such systems over the past two decades. The coverage has been limited specifically to applied sys­ tems, or areas with potential applications, although over 500 references cite much of the literature on the fundamentals of the subject. In general, non-biological organic photochromism in organic matrices has been covered. However, the unique properties of polysiloxanes merit special mention in Chapter 4, because of the attributes that such inorganic polymers can provide in certain systems such as liquid crystalline photochromic polymers, where two extremely interesting phenomena are combined. In addition to outlets in polarization-sensitive holographic recording media, such materials exhibit interesting non-linear optical effects suitable for optical switching and rheo-optical phenomena which may find application in mechano-optic transduction. Within this framework examples of all the important photochromic mechanisms are covered by authors from both the industrial and the academic sectors. Given the photonic nature of the phenomenon under discussion, it is not surprising that many optical applications have been proposed. It is perhaps more surprising, however, that, until recently, no large scale markets had been identified that could commercially exploit photochromic phenomena.