FOLLOWING a brief review of the history of HEMTs different application areas are presented. Those are split in three major groups: RF communication systems, automotive electronics, and sensors. The advantages of complementary normally-off structures is discussed and several approaches to achieve such operation are described. Reports of different research groups during the last decade are summarized also in respect of currently available commercial products. HEMT structures based on other materials than GaN are shortly discussed followed by a brief summary of reports on GaN-based HBTs. The chapter ends with a review of various simulation software packages and an introduction of our device simulator MINIMOS-NT.