Electrochromic materials are able to change their optical properties in a persistent and reversible way under the action of a voltage pulse. This book explores electrochromism among the metal oxides, with detailed discussions of materials preparation (primarily by thin film technology), materials characterization by (electro)chemical and physical techniques, optical properties, electrochromic device design, and device performance. The vast quantity of information presented is structured in a systematic manner and the optical data is interpreted within a novel conceptual framework.The publication will serve as a comprehensive foundation and reference work for future studies within the rapidly expanding field of electrochromic materials and devices. These devices are of particular interest for information displays, variable-transmittance (smart) windows, variable-reflectance mirrors and variable-emittance surfaces.