Photonics,whichusesphotonsforinformationandimageprocessing,islabeled as one of the technologies of the 21st century, for which nonlinear op- cal processes provide the key functions of frequency conversion and optical switching. Chalcogenide glasses are based on the chalcogen elements S, Se, and Te. These glasses are formed by the addition of other elements such as Ge, As, Sb, Ga, etc. These glasses are low-phonon energy materials and are generally transparentfromthevisibletoinfrared.Chalcogenideglassescanbedopedby rare-earth elements such as Er, Nd, Pr, etc., and hence numerous applications ofactiveopticaldeviceshavebeenproposed.Theseglassesareopticallyhighly nonlinear and could therefore be useful for all-optical switching. This book is a review of recent progress in the science and technology of chalcogenide glasses, with an emphasis on their nonlinear optical properties, forgraduatestudents,practisingengineersandscientistsfromawidemultid- ciplinary area such as physics, chemistry, electrical engineering and material science. Since the interest in this area is growing worldwide, a book dealing with this subject will be of great value to researchers of varied backgrounds. Chalcogenide glasses and their electronic, structural, and photoinduced properties are introduced. Techniques to characterize the linear and nonlinear optical properties of these glasses are introduced and used to measure the optical constants of chalcogenide glasses in the form of bulk, thin ?lm and ?ber. The possibilities of fabricating passive and active devices are presented.