Optical communication systems use light to transmit information from one place to another. Light is a type of electromagnetic radiation like radio waves. Today, infrared light is being used increasingly as the carrier for information in a communication system. The transmission medium is either free space or a special light-carrying cable called a fiber-optic cable. Because the frequency of light is extremely high, it can accommodate very high rates of data transmission with excellent reliability. This chapter introduces the basic concepts, circuits and the most widely used optical networking technologies.