This chapter introduces modern digital and wireless communication systems. This introductory chapter introduces a basic point-to-point physical communication system to illustrate the basic partitions of a communication system, such as source encoding/decoding, channel encoding/decoding, and modulation/demodulation. The essential component of any physical communication system is the communication channel and its mathematical modeling. These include the additive white Gaussian noise (AWGN) and linear filter channel models. The chapter introduces the concept of a coding channel, defined as the portion of the communication system that is seen by the error control coding system. Communication systems detect communication signals in the presence of noise, and the notions of signal-to-noise ratio and information bit energy-to-noise ratio are introduced. Finally, the chapter introduces the concept of Shannon channel capacity, defined as the maximum possible information bit rate that can be reliably transmitted over a specified coding channel.