Transmission lines in communication carry telephone signals, computer data in LANs, TV signals in cable TV systems, and signals from a transmitter to an antenna or from an antenna to a receiver. Transmission lines are critical links in any communication system. They are more than pieces of wire or cable. Their electrical characteristics are critical and must be matched to the equipment for successful communication to take place. Transmission lines are also circuits. At very high frequencies where wavelengths are short, transmission lines act as resonant circuits and even reactive components. At VHF, UHF, and microwave frequencies, most tuned circuits and filters are implemented with transmission lines. This chapter covers basic transmission line principles—theory, behavior, and applications.