This book on gallium nitride (GaN) and associated materials focuses on advances in basic science, as well as the rapidly maturing technologies involving blue/green light emitters, detectors and high-power electronics. A highlight is a report on wide-bandgap semiconductor research done in Europe. Also reported is the commercialization of a laser operating at 405nm wavelength with a 4000-hour device lifetime. At 450nm emission wavelength, significant reductions in lifetime were found, and are believed to arise from nonideal properties of the InGaN alloy used in the active layer of the device. Additional topics include: the significant success of transistors for microwave applications; improvements in the epitaxy of GaN, using both selective area growth techniques (lateral epitaxy overgrowth) and introducing low-temperature intralayers in the films; advances in both molecular beam epitaxy and metal-organic vapor phase epitaxy, including several studies of quantum dot formation in strained alloys and improvements in hydride vapor phase epitaxy, particularly for providing very thick films.