Before we consider the scattering from a rough sea surface, we start with the case of a plane interface, which serves as a reference for the latter. We consider Maxwell’s equation in a conducting medium instead of the vacuum, whose electrical properties are described by the complex permittivity and impedance of seawater. We introduce the canonical horizontal and vertical EM polarization, which describes the orientation of the EM field relative to the reference plane and plays a major role in the reflective properties of the surface. We recall the definition and properties of the Fresnel reflection coefficients, which gives a first understanding of why HFR use only vertically polarized electric fields.