The "blue laser" is and exciting new device used in physics. The potential is now being recognised for its development into a commercial lighting system using about a tenth of the power and with a thousand times the operating lifetime of a comparable conventional system. This comprehensive work introduces the subject at a level suitable for graduate students. It covers the basics physics of light emitting diodes (LEDs) and laser diodes (LDs) based on gallium nitride and related nitride semiconductors, and gives an outline of their structural, transport and optical properties, and the relevant device physics. It begins with the fundamentals, and covers both theory and experiment, as well as an examination of actual and potential device applications. Shuji Nakamura and Nichia Chemicals Industries made the initial breakthroughs in the field, and these have revealed that LEDs and LDs are a sophisticated physical phenomenon and a commercial reality.