The goal of the physical layer is to send a stream of bits between two devices connected by a communication channel. The communication channel can either be a wire connecting the devices (fiber or copper) or a wireless “link” in free space. While the fundamental concepts underlying communication are the same for wireless and wired channels, there are many unique challenges associated with wireless communication. This chapter provides an informal, high-level overview of wireless communication. Our goal is twofold. First, the chapter provides the foundation needed to explain the properties of the wireless technologies presented in the next two chapters. Second, we will discuss how the unique challenges associated with wireless communications channels impact network performance in different physical environments. More detailed information on the physical layer can be found in several textbooks on wireless communication and networking (Molisch, 2022); (Beard, C., & Stallings, W. (2016)).