ABSTRACT III-nitride (III-N) devices are feasible for next-generation harsh environment applications owing to their unique material properties. With low resistive loss and suitability for high-voltage operation, III-N devices are actively being developed for high-power microwave amplifications and highvoltage power switching with operating temperatures beyond 200°C. These harsh environment applications include downhole drilling, electric vehicles, on-board engine control systems and the outdoor power grid. This chapter provides an introductory review of the development of III-N electronic devices. A brief overview of the material fundamentals of III-N semiconductors and design considerations for polar semiconductor devices is presented. State-of-the-art III-N high-electron-mobility transistors (HEMTs) for highpower microwave/millimetre-wave and high-voltage switching, as well as novel III-N bipolar switches, including heterojunction bipolar transistors (HBTs) and rectifiers, are discussed to exploit the potential of III-N devices in ultra-high-power and high-temperature applications. Finally, a summary of the radiation effect study on III-N materials and devices is presented.