Gallium nitride (GaN) is a binary III/V direct bandgap semiconductor commonly used in bright light-emitting diodes since the 1990s. Its sensitivity to ionizing radiation is low, making it a suitable material for solar cell arrays for satellites. This book covers GaN from the fundamental physics to the fabrication of devices and circuits that are already foreseen to replace standard silicon or gallium arsenide (GaAs) in many applications.