In this chapter, the operating principles of an ideal hot electron transistor (HET) will be introduced, illustrating the device’s direct current (DC) characteristics and discussing the impact of the main physical parameters on the DC figures of merit (current transit ratio and current gain) and on the alternating current (AC) figures of merit (f max and f T ). Therefore, an historical perspective on the attempted implementations of this device concept will be provided, starting from the first proposal of a metal base HET to more recent implementations, such as the nitride semiconductors–based HETs and the Gr base HET (GBHET). The theoretical DC and AC performances of GBHETs will be discussed and state-of-the-art GBHETs will be presented. The last section of the chapter will present open issues and new ideas to improve the performances of GBHETs.