We review advances in GaN-based microwave power field-effect-transistors (FETs). Evolution in device technology included metal-semiconductor-field-effect-transistors (MESFETs), heterostructure-field-effect-transistors (HFETs), modulation-doped-field-effect-transistors (MODFETs) or high-mobility-transistors (HEMT), HEMTs with high Al contents, HEMTs with gate recess and GaN-channel HEMTs grown on SiC substrates. The power density was first reported as 1.1 W/mm at 2 GHz using an AlGaN/GaN HEMT structure grown on sapphire substrate, and was subsequently improved to 1.5-1.7 W/mm at 4-10 GHz by refinement in device structure and processing techniques. This was advanced to 2.6-3.3 W/mm at 8-18 GHz by adopting a high-Al-content AlGaN barrier layer. Success in gate recess helped to further increase the power density of these GaN HEMTs on sapphire substrates to 4.6 W/mm at 6 GHz. Substrate replacement of sapphire by SiC, for excellent thermal dissipation, has boosted performance to 6.9 W/mm at 10 GHz, which is higher than GaAs-based FETs by a factor of 6. Device periphery was scaled up to obtain high total output power. On one hand, GaN HEMTs on sapphire, using a flip-chip bonding technology for thermal management, have generated 7.6 W at 4 GHz. On another hand, GaN HEMTs on SiC, taking advantage of the high substrate thermal conductivity, have achieved 9.1 W at 7.4 GHz. Two types of initial GaN-based power amplifiers were also demonstrated using a flip-chip IC scheme. The transistors used were 0.7 to 0.8-µm-long-gate GaN HEMTs. Bandwidths of 1-8 GHz and 3-9 GHz were achieved with gains up to 11.5 dB. The output power levels ranged from 3.2 to 4.6 W using devices with 2 and 3-mm gate peripheries, which were higher than that achievable with GaAs-based HEMTs of the same size by a factor of 2. Traps in the device structure currently limit performance of most GaN FETs. These traps cause dispersion in the I-V characteristics, which increases knee voltage and reduces channel current under RF gate drive. However, they are believed to be not inherent in the GaN semiconductor system and can be minimized as the technology matures.