In recent years explosion seismology has produced much new knowledge about the nature of the crust-mantle boundary, and the materials that it separates in the lower crust and upper mantle. On the one hand, it is clear that both the lower crust and upper mantle contain more fine structure than was previously imagined and, on the other, that variations in deep structure correlate to some extent with surface geology. For example, a thin crust and sharp Moho are characteristic of Hercynian Europe whereas shield areas and orogenic belts are characterized by a thick crust and gradational Moho. Furthermore useful seismic constraints are now emerging regarding the materials to be found in the crust and mantle. For example, it is clear that the immediate sub-Moho material is most probably peridotite.