Publisher Summary This chapter focuses on crystal structures for bulk-like Ni oxide, and preparation and characterization of films made by evaporation, sputter-deposition, and by electrochemical and chemical methods. It provides the discussions of ion intercalation/deintercalation reactions and diffusion constants, analyses of the intercalation/deintercalation by electrochemical and physical techniques, and data on ultraviolet absorption and the semiconductor bandgap. It describes the optical properties in the luminous and near-infrared wavelength range specifically, for films made by evaporation, sputter-deposition, and by electrochemical and chemical techniques. It concludes with discussions on coloration efficiency and some notes on theoretical models for the electrochromism. Reactive evaporation is a widely used and studied technique for making Ni oxide films. Reactive evaporation of NiO powder in the presence of O2 can be viewed as the standard method; it yielded deposition rates up to ∼1 nm/s.