Light is emitted in semiconductor materials as a result of electron transitions from the conduction band to recombine with a hole in the valence band, thereby releasing energy in the form of a photon. This emission can occur through either spontaneous or stimulated emission, as discussed in the previous chapter. Semiconductor light sources such as light-emitting diodes (LEDs) are based on spontaneous emission, whereas laser diodes (LDs) utilize the property of stimulated emission. These light sources are typically based on pn-junction diodes under a forward bias condition. The electric current in the device injects electrons and creates an excess carrier concentration, leading to radiative recombination and the subsequent emission of photons. This chapter provides a comprehensive overview of their operation, characteristics, and emerging trends. We discuss their theory of operation, input-output relationships, output characteristics, and device structures.