In recent years, various transparent conductive oxide (TCO) electrode materials that serve as alternatives to ITO in optoelectronic devices have attracted considerable attention, one of which is gallium-doped zinc oxide (GZO). GZO is used in electrode fabrication due to its high transparency in the visible light spectral region, i.e., exceeding 90%, and its low resistivity, i.e., below 10–4 ohm · cm. In addition, it can also be applied in ultraviolet detection because of its wide bandgap and direct bandgap characteristics. This chapter first introduces the fundamentals of GZO thin film materials, including discussion on the basic electrical and optical properties of GZO films. It then systematically presents how the preparation conditions of GZO films affect their optical and electrical properties. Subsequently, the chapter explores the application of GZO films in device fabrication, focusing on novel and interesting device characteristics and influence of microstructures of the GZO films on device performance.