Electrochromic windows, windows that can automatically increase or decrease the amount of visible light entering a building, represent an exciting new technology that promises both energy savings and occupant amenities by enhancing outside view in high-glare sun orientations. Both organic and inorganic materials have widely recognized for their fruitful applications in light-attenuating smart windows and displays. This book provides a comprehensive treatment about the fundamentals of electrochromic systems and their many applications. It is mainly focused on updated comprehensive review of electrochromism in niobium oxide thin films (Nb2O5 particularly). The book comprises studies on synthesis of pristine and doped niobium oxide thin films by simple and inexpesive spray pyrolysis technique.