In this chapter, III-V compound semiconductors MESFET, HBT, and HEMT are described, including papers which report major achievements of the HEMT technologies in the fields of microwave, millimeter-wave, and digital Integrated Circuits (ICs). The important aspects of device physics, small-signal equivalent circuits for GaAs, and GaN-based HEMT are discussed. The authors present a comparative analysis of different analytical modeling techniques and show that the differences reflect the physical and technology differences of the tested microwave transistors. The purpose of this chapter is to facilitate the choice of the most appropriate strategy for each particular case. For that, the authors present a brief but thorough comparative study of analytical techniques developed for modeling different types of advanced microwave transistors: GaAs HEMTs, GaN HEMTs. The chapter shows that a crucial step for a successful modeling is to adapt accurately the small-signal equivalent circuit topology.