So far we have only considered the propagation of EM waves above a plane air/seawater interface, which is governed by the specular reflection and the interference with the incident field. In presence of surface roughness, such as that induced by the gravity waves, the electromagnetic field is scattered in various directions of space, in particular in the backscattering direction where it can be measured by the radar. The description of the scattered field in the context of HFR is a special case of rough surface scattering, a broad field of research that includes optical, acoustic or radio wave scattering and which can be described within a common mathematical framework. In this chapter, we recall the main notions of the theory of rough surface scattering and we introduce the perturbation expansion which gives the scattered field as a function of the sea surface roughness in the case where the radar wavelength is much larger than the wave height. We introduce the radar and Doppler cross sections, which are the primary observables of an HFR experiment, and relate them to the ocean wave spectrum. We introduce the notion of effective surface impedance, which allows to quantify the attenuation by surface roughness in the ground wave propagation, and obtain some simple estimates for the backscattered power as a function of the distance to the radar.