Scattering of electromagnetic (EM) waves from the sea surface is usually considered with the help of a two-scale (composite surface) model. It is shown theoretically that for broad wavelength range at grazing angles less than about 20¿, diffraction of the incident field on large-scale (undulating) components cannot be considered using the tangent plane (Kirchhoff) approximation. In other words, an undulating surface cannot be treated as a set of locally plane facets, and its curvature becomes an important parameter. The effects of curvature of the undulating surface are estimated for a two-dimensional situation. It is shown that in this case, backscattering cross sections at low grazing angles are proportional to the sin2 α rather than to the sin4 α (which would be the case according to Rice's classical formulae). This corresponds to experimental results. The theoretical curve of the &sgr;HH/&sgr;VV ratio fits well the experimental data published by Lee et al. <1995> (for ''slow'' signals). ¿ American Geophysical Union 1996