When viewed at extremely shallow grazing angles with a microwave radar, sea backscatter displays a variety of spatio-temporal features with time scales of the order of several minutes or more. A modest body of experimental evidence accumulated from several radars looking at different oceans shows components with a period of 3–4 min sometimes imposed on a much stronger periodicity of 10–15 min, and sharply defined moving striations with speeds of the order of 15 kts and lifetimes of several minutes. The origin of these features may be sought among a wide assortment of physical phenomena, including surface effects (such as wave packets or interference patterns, illumination thresholding at extreme grazing angles, internal waves in the thermocline, shelf waves, fluctuations in the surface wind field), or propagation effects resulting from refractivity perturbations due to traveling disturbances in the atmospheric boundary layer or to water vapor perturbations in the evaporative layer. None of these hypotheses can account for all of the features observed in the data, but the more likely candidates are discussed in detail, using available data for the relevant oceanographic and atmospheric parameters. It is concluded that the behavior shown in this paper is most probably due to group effects among surface waves emphasized by illumination thresholding at low grazing angles.