In Belgium not much is known about the extension of the different crustal structures into the middle and lower crust. Until now, only seismic profiles and gravimetric and magnetic data gave some insights. The general aim of this thesis is to improve the understanding of the lower/middle crustal structure of Belgium. This will be achieved by determining the Moho depth and performing a local seismic tomography. The Moho is rather flat underneath Belgium with Moho depths ranging from 31 km underneath the Brabant Massif (BM) over 31-33 km underneath the Roer Valley Graben (RVG) and the Ardennes Allochthon (AA) to 28 km underneath the Eifel Volcanic Province (EVP). These results are comparable to the results of other studies, like seismic reflection profiles and receiver function analysis. The degree of precision obtained underneath the Belgian onshore part of the BM, however, has not been reached before. At the southeast of Grand-Duchy of Luxemburg, two seismic reflection phases are observed on the seismograms. The origin of the first reflection (at 15 to 24 km) is not clear yet, but this problem will be solved with the results of the seismic tomography. The second reflection is interpreted as the current Moho interface. The local seismic tomography confirmed the Moho uplift to 28 km of depth underneath the Campine Bassin and the EVP. Furthermore, the inversion enables to determine the cause of the intra-crustal reflection mentioned in the previous paragraph. Lower crust velocities are observed at 13.5 km depth at this location. The reflector represents thus a lower crust/upper mantle being upthrusted in the middle crust during the Variscan Orogeny. Superficial geology is also recognizable, like the BM, the AA, the Rocroi and Stavelot Inliers, the sediment infill of the RVG and the Trier Graben. There is a velocity contrast over the entire crust between the AA and the BM. The cause of this contrast is not clear, yet. It either is a crustal separation or a mid-crustal deformation due to the Variscan Orogeny. ...