© Springer Science+Business Media Singapore 2016. Light-emitting diodes (LEDs) have been exploited from laboratory plant researches to commercial cultivations in greenhouses or plant factories with artificial lighting (PFAL). Benefits of LEDs for plant cultivation are their ability to emit thermal-free monochromatic light, solidity, minute size, minimal electricity consumption, longevity, and controllability of light emission. Varieties of LED lamps are available in the market, specialized respectively for PFAL, greenhouse, and laboratory applications. Those commercial products are well designed and useful, but users also can design and fabricate crop-specific LED lighting systems according to their purposes. To maximize the merits of LEDs for plant cultivations, the knowledge about fundamental principles of LED engineering would help growers. In this chapter, the light emission from a semiconductor p-n junction, the regulation of emission, heat treatments, and the regulation of light distribution on a culture bed are described.