It was quite difficult to grow high quality single crystals of nitrides and impossible to control their electric conduction. These problems prevented the development of nitride-based devices for many years. In 1986, a dramatic improvement in the crystalline quality of GaN was achieved by use of low-temperature-deposited (LT) buffer layer technology in metalorganic vapor phase epitaxy. In 1989, the high quality GaN enabled us to produce p-type conduction in nitrides and to control the conductivity of n-type nitrides. These achievements led to the invention of the world's first GaN p-n junction blue/UV LED in 1989. Room temperature UV stimulated emission, which is indispensable for laser operation, was also achieved in 1990 by use of high quality GaN films grown with the LT-buffer layers. These breakthroughs inspired nitride researchers around the world to greater efforts, and eventually led to the commercialization of high-performance blue LEDs and long-life violet LDs as well as the development of nitride-based devices such as high-speed transistors. Furthermore, unique properties such as a large piezoelectric effect were also clarified due to the marked improvements in crystal quality of nitrides. In this paper, key inventions during the development of nitride-based blue LED and LD are reviewed and a recent advance in UV devices is also described. (c) 2006 Published by Elsevier B.V.