This piece of work was realised within the framework of the ECORS programme. its aim was to study the crustal structures of the Western Pyrenees and their relationship with the sedimentary basins. This required the gathering oi seismological data (teleseismic residuals, local seismicity) and the compilation of gravimetric data. The first part deals with the structural survey of the Western Pyrenees by using different models of gravimetrical, seismic reflection and seismological observations The resulting structural model is: in the Iberic part, the Moho is about 40-50 km deep and, with a sharp transition, goes to a normal depth of 30 km in the European part; this transition is characterized by an intra-crustal body which has a high density (2 900 kg/m(3)) and a high velocity (7 000 mis). In the second part, we studied the relationship between the Mesozoic basins and the later evolution of the Pyrenees. Using the subsidence and sedimentary history oi the Arzacq sub-basin, this study leads us to propose an opening model. Secondly, the sedimentary and thermal history is shown by a 2D numerical model. Post-rift thermal relaxation localizes deformation on the edges of the rift because of the lateral thermal transfers which correspond to the tectonic evolution during the Pyrenean compression.