This chapter serves as an introduction to the chapters on III-nitrides in this book. It gives a brief review of the development of relevant III-nitride materials for light emitters since the late 1960´s, when single crystalline GaN layers grown on sapphire were first demonstrated. The first wave of scientific work died out in the late 1970´s, since low-ohmic p-GaN could not be made at the time. After another 10 years several important breakthroughs were made, using the technology of metal organic vapor phase epitaxy (MOVPE). Smooth thin epilayers could be made, and ways to dope the materials n-type as well as p-type were invented. In the period 1986-1997 high brightness violet and blue double heterostructure (DH) LEDs, narrow quantum well (QW) LEDs, and QW based violet laser diodes with a long operating lifetime of 10000 hours were demonstrated, mainly by Japanese groups. Since then the development efforts have spread worldwide, and a large spectrum of novel applications based on nitride emitters are already in practical use. Perhaps the most important one is the future possibility of using nitride LEDs for general lighting purposes.