The operating range of HFR goes well beyond the line of horizon, a remarkable property that has been known and used for decades in defense applications and justifies the common denomination of Over-The-Horizon (OTH)Over-The-Horizon radar radars. The physical origin of the long distance propagation is the presence of an electromagnetic ground waveGround wave, also called surface waveSurface wave (not to be confused with the gravity waves!). This is a component of the EM field which remains localized close to the interface and undergoes little attenuation as it propagates away from the source, following the Earth curvature. It adds up with the ordinary EM waves, called the space wavesSpace wave, which propagate in the atmosphere in direct path from the transmitter or after reflection by the sea surface. Surface waves can propagate two ways after scattering from an object or from the gravity waves (Fig. 8.1), which makes them very useful for many remote sensing applications. There has been a longstanding controversy in the literature as to the nature of surface waves. We refer to [48] for an excellent historical survey and a clarification of the concept. Today, the mechanism of ground wave propagation is well understood. In this chapter, we review briefly the main steps that lead to its correct formulation. A more detailed analysis can be found in classical papers [149, 200–202, 272] and textbooks [145, 165, 190, 271]. This allows us to estimate the basic transmission loss, which quantifies the attenuation of the electric field as it propagates away from the transmitting source.