Abstract: Owing to its exceptional physical and electronic properties, gallium nitride (GaN) is a promising material that can find application in the fields of high-power and high-frequency electronics. However, there are still several hurdles in GaN technology, which hinder the full exploitation of the great potential of this material. For that reason the scientific community working on GaN-based materials is continuously involved in addressing a variety of physical and technological problems encountered in the fabrication of GaN devices. This paper aims to give an overview on some selected scientific problems related to GaN technology for power electronics devices, with a special attention to the case of high electron mobility transistors (HEMTs). In particular, after an introduction on the fundamental physical properties of the material, special emphasis is given to the problem of current transport at metal/GaN interfaces, considering both Ohmic and Schottky contacts. Afterwards, the importance of dielectrics either as passivation or gate insulation layers is briefly highlighted. Then, the possible approaches to control the two-dimensional electron gas (2DEG) in AlGaN/GaN heterostructures and to fabricate normally-OFF HEMTs are presented. Finally, a short description of the status of vertical GaN devices technology is given.