Light emitting diodes (LEDs) and laser diodes are the focus of this chapter. For LEDs materials choices, the concepts of internal and external quantum efficiency as well as device design are treated. Special devices such as white LEDs, quantum dot and organic LEDs are introduced. For laser diodes the concepts of gain, loss and threshold, various heterostructures for modern device design and laser emission properties such as mode spectrum, far field, dynamics and tunability are discussed. Finally special devices such as the hot hole laser, the cascade laser and semiconductor optical amplifiers are mentioned.