This chapter provides a broad overview of microwave annealing of semiconductors with a focus on the specific case of gyrotron-microwave annealing of Mg-implanted GaN. The advantages, drawbacks, and considerations of pairing an appropriate microwave source for a given semiconductor are discussed. The ability to achieve high-conductivity GaN implanted with Mg or Mg/N using a 1-minute anneal using a 60 GHz gyrotron microwave source is shown. Protection of GaN against degradation in nonequilibrium 1450 °C annealing conditions using a combination of protective AlN cap, 3 MPa N2 overpressure, and few-second heating cycles is described. The annealing of GaN co-implanted with N and Mg is shown to enhance achievable hole concentrations by removing implant-induced VN defects. The challenges associated with implanted dopant diffusion and the resultant distortion of the implanted profile, as well as a multi-step, multi-temperature annealing methodology to suppress diffusion by an order of magnitude, is presented.