GaN-based devices for both high efficiency switching and RF applications in modern power electronics are increasingly moving into the focus of worldwide research and development activities. Due to their unique material properties GaN power devices are distinguished by featuring high breakdown voltages, high electron mobility, low on-state resistances and fast switching properties at the same time. These devices show promise for both existing as well as emerging applications. On the other hand, to fully exploit the outstanding GaN-based semiconductor properties, the material itself has to be stressed significantly during standard device operation, which lead to reliability issues such as DC to RF dispersion and dynamic on-state resistance degradation. Thus, material quality, the specific epitaxial design as well as the device topology will directly influence device performance, reliability and mode of degradation. GaN-based device concepts to address the specific needs of each of these areas are under intensive development by groups around the world. The first part of the tutorial will describe why GaN is an attractive material for RF and power electronic applications. Then, this tutorial will provide an overview of the material properties, device structures, and fabrication processing issues surrounding this emerging device technology. It will especially discuss technological ways towards engineering both high performances together with reliability into these devices.