Abstract Thick epitaxial GaN power switching devices are known to contain a high density of crystal defects, especially threading dislocations in the epitaxial layer. The impact of these defects on device performance is a topic of ongoing research. This paper presents a review on the current understanding of the impact of dislocations on leakage current, blocking voltage, peak electric field, switching frequency, and dynamic ON resistance. The effects of substrate type on dislocation density are also discussed for optimization of switching two terminal devices.