Shuji Nakamura’s development of a blue semiconductor laser on the basis of Ga N opens the way for a host of new applications of semiconductor lasers. The wavelengths can be tuned by controlling the composition. For the first time it is possible to produce lasers with various wavelengths, ranging from red through yellow and green to blue, in one substrate material. This fact, together with their high efficiency, makes Ga N-based lasers very useful for a wide spectrum of applications. The second edition of this basic book on Ga N-based devices has been updated and significantly extended. It includes a survey of worldwide research on Ga N, as well as Nakamura’s latest important developments. The reader finds a careful introduction to the physics and properties of Ga N. The main part of the book deals with the production and characteristics of Ga N LDs and LEDs. To complete the spectrum of applications, Ga N power devices are also described.