Over the past few years, systems based on gallium nitride high-electron-mobility transistors (GaN HEMTs) have increasingly penetrated the markets for cellular telephone base stations, RADAR, and satellite communications. High power (several W/mm), continuous-wave (CW) operation of microwave HEMTs dissipates heat; as the device increases in temperature, its electron mobility drops and performance degrades. To enhance high-power performance and enable operation in high ambient temperature environments, the AlxGa1[-]xN/GaN epitaxial layers are attached to polycrystalline diamond substrates. e lower surface temperature rise on GaN-on- diamond is directly measured; subsequently, improved electrical performance is demonstrated on diamond versus the native (Si) substrates. Benchmark AlxGa1[-]xN/GaN devices are fabricated on SiC for comparison to diamond, Si, and bulk GaN substrates; the merits and performance of each is compared. In collaboration with Group4 Labs, X-band amplifier modules based on GaN-on-diamond HEMTs have been demonstrated for the first time. Recent efforts have focused on substituting AlxIn1[-]xN barriers in place of AlxGa1[-]xN to achieve higher output power at microwave frequencies and addressing the challenges of this new material system. Ultimately, these techniques may be combined to attain the utmost in device performance.