A review of the effects of proton, neutron, γ-ray and electron irradiation on GaN materials and devices is presented. Neutron irradiation tends to create disordered regions in the GaN, while the damage from the other forms of radiation is more typically point defects. In all cases, the damaged region contains carrier traps that reduce the mobility and conductivity of the GaN and at high enough doses, a significant degradation of device performance. GaN is several orders of magnitude more resistant to radiation damage than GaAs of similar doping concentrations. In terms of heterostructures, preliminary data suggests that the radiation hardness decreases in the order AlN/GaN >AlGaN/GaN > InAlN/GaN, consistent with the average bond strengths in the Al-based materials. The radiation damage can alter electric field profiles and drain current due to trapped negative charge and lead to increased breakdown voltages and apparent reliability. Simulations of this effect show good agreement with experiments. We also discuss the literature on effects of different radiation types on the performance of GaN-based high electron mobility transistors.