The aim of this study is to evaluate the sensitivity of Lg waves to lateral changes of the earth's structure. Considering a simple model of uplift of the Moho, numerical simulations show that the geometrical attenuation of Lg is not much affected by a smooth anomaly of the Moho depth. On the other hand, the passing of the Lg wave through the region of the Moho uplift results in a clear deterioration of the wave shapes, which confirms the occurrence of mode conversions. The presence of an overlying sedimentary basin causes a local amplification of Lg above the basin itself and the appearance, behind the basin, of a secondary surface wave guided in the sediments. The effect of the basin on the Lg wave is found to be reasonably taken into account by applying a local amplification function to the data. We use a data set consisting of records of Lg phases in France to test the conclusions of our numerical study. The mapping of the apparent quality factor, computed from Lg at different frequencies, confirms the interpretation of the strong attenuation of S waves around 1 Hz in terms of scattering and shows the weak sensitivity of the amplitude of Lg to smooth changes in the depth of the Moho. ¿ American Geophysical Union 1987