In this chapter, we begin by discussing the fundamentals of the radio channel. Typical Radio Transmitter and Receiver block diagrams are reviewed, together with the components of a wireless communications system. Then, we turn to addressing the phenomenon of wave propagation, in particular, Multi-Path Propagation, Free-Space, and Path Loss. The relationship between power and electric field is elucidated, the Effective Isotropic Radiated Power is defined, and the measured received electric field is related to the received power. Then, the various models invoked in calculating the effects of propagation are presented. These include: Reflection, in particular, the conditions of: (1) Two Half-Spaces: Dielectrics; (2) Two Half-Spaces: Metals; (3) Two Half-Spaces: Multi-Layer Structures; and (4) Two-Ray Ground Model. This is followed by a discussion of Diffraction, in particular, from a Knife-Edge. We conclude this discussion with the topics of Scattering, and Reflection in the Troposphere. The chapter is concluded with a discussion of antenna parameters, in particular: (1) Efficiency; (2) Effective Area; (3) Gain; and (4) Directivity.