In the last decade, GaN has emerged as an excellent material for application in power electronics. The wide energy gap of gallium nitride (3.4 eV) enables high-temperature operation, while the large breakdown field (3.3 MV/cm, 11 times higher than silicon) allows to reach kV-range operation while maintaining a low on-resistance. Thanks to the high mobility of the two-dimensional electron gas at the AlGaN/GaN interface, lateral GaN transistors have a very low on-resistance; commercial products are already on the market, and new technologies are being developed, targeting the 650 V/900 V ranges. Recently, the attention toward vertical GaN transistors has constantly increased: a vertical layout allows to substantially increase current density while taking advantage of the wide bandgap and high breakdown field of GaN. Vertical transistors based on GaN are expected to find application in converters with medium high power levels (>5 to 10 kW), once the technology reaches maturity. This chapter reviews the main properties of GaN-based materials and devices. The first part focuses on the main challenges related to the growth of high-quality GaN epitaxial stacks. The second and the third parts focus on GaN lateral and vertical devices, respectively. Finally, the fourth section describes the trapping and degradation processes that limit the performance and the reliability of GaN transistors, and the related physical mechanisms. This chapter represents an ideal guide for a reader willing to enter the world of GaN devices.