Results of deep seismic soundings from various areas of the world are presented. Much weight is put on recent European investigations. Seismic refraction surveys are found to yield reliable information on the depth and undulation of the Mohorovicic discontinuity (Moho). Wide angle reflection data are used for the derivation of an overall velocity gradient zone of a few kilometers in many continental areas. From near vertical and subcritical reflection data a stepwise character of the gradient zone is deduced. The overall picture from different seismic investigations shows the Moho as a laminar transition zone of a few kilometer thickness. There is a general stepwise increase in the velocity of compressional (P) and shear waves (S), possibly often interrupted by layers with lower velocities, until values around 8 km/s are reached for theP-waves in the uppermost part of the mantle. Petrologic explanations of these features deal with layers of partial melts, crystallisation seams, intrusions and some peeling of mantle matter. Problems of the evolution, shifting, and subduction of the Moho are discussed in the light of plate tectonics.