Over the last few years, hydrated nickel oxide or hydroxide has gained interest as an electrochromic material. It has many important features. First it shows a transparent to bronze coloration which isssuitable for a number of architectural, vehicle and aerospace glazing applications. This material is stable insan alkaline environment and colors and bleaches at potentials below 1.0V and below the oxygen andshydrogen evolution potentials. It can be deposited by a wide range of processes, including anodic andscathodic electrodeposition, r.f. and d.c. sputtering, vacuum evaporation, chemical vapor deposition, andssolgel processes. In this work we will discuss films made chiefly by anodic electrodeposition. Some of thesbest characteristics are seen in films made by electrodeposition. The properties of films made by othersdeposition processes will be discussed in subsequent chapters. Much of the electrochemical knowledgesabout hydrated nickel oxide comes from studies of metallic nickel electrodes used in batteries. The studysof hydrated nickel oxide based electrochromic devices is just emerging, and so far very few devices havesbeen reported. Some of the work by my group on nickel based devices will be outlined later in thisschapter.