During the past 40 years, light-emitting diodes (LEDs) have undergone a significant development. The first LEDs emitting in the visible wavelength region were based on GaAsP compound semiconductors with external efficiencies of only 0.2 %. Today, the external efficiencies of red LEDs based on AlGaInP exceed 50 %. AlGaInP semiconductors are also capable of emitting at orange, amber, and yellow wavelengths, albeit with lower efficiency. Semiconductors based on AlGaInN compounds can emit efficiently in the UV, violet, blue, cyan, and green wavelength range. Thus, all colors of the visible spectrum are now covered by materials with reasonably high efficiencies. This opens the possibility to use LEDs in areas beyond conventional signage and indicator applications. In particular, LEDs can now be used in high-power applications thereby enabling the replacement of incandescent and fluorescent sources.