Communication transmission channelstransmission channel are the heart of transmission systems: They establish a connection between a transmitter and a receiver, which allows information to be transmitted between the two endpoints via physical-technical means. Therefore, they are often at the center of communications engineering considerations or they form the starting point for the design of transmission systems, with the help of which information is to be transmitted over a given channel. After a brief general introduction to transmission channels and their classification, the focus of this chapter is on the description of copper lines as a channel for communication transmission. Starting from the electrical properties of copper two-wire lines, a system-theoretical description of the transmission channel copper cable is developed via the transmission line theory, which results in a cable transfer function and which is suitable and appropriate for the analysis and optimization of transmission systems that use such channels for information transmission. Finally, special types of couplings in communication cables are examined and also described system-theoretically, which result from the constructive structure of such cables and which are referred to as crosstalk.