The seismic structure of the crust of the NW Iberian Peninsula, along the Cantabrian Mountains and their southern foreland, the Duero basin, is investigated from the first data set available from refraction and wide-angle reflection profiles. The velocity-depth distributions obtained along E-W and N-S transects evidence lateral variations in crustal structure that can be associated to Variscan and Alpine tectonic episodes. The western part of the range, toward the Variscan hinterland zones, as well as the Duero basin below the sediments, show a crustal structure similar to that of the Variscan belt elsewhere in Europe. The total crustal thickness is 30--32 km, and three main layers (upper, middle, and lower crust with average velocities 6.0, 6.25, and 6.8 km/s) are resolved. Some Paleozoic structures such as the basal detachment of the Cantabrian Zone are expressed in the seismic models. The crust appears to vary eastward in the external zones. The most outstanding feature is a crustal root beneath the highest Cantabrian summits where the Moho is found at 47 km depth. A prominent Alpine reworking of the crust in the eastern part of the Cantabrian Zone is thus revealed, and the N-S seismic transects delineate a geometry similar to the one found across the central Pyrenees in the Etude Continentale et Oc¿anique par Reflexion et Refraction Sismique (ECORS) profile, revealing the importance of the Alpine deformations in the northern part of the Iberian plate. ¿ 2000 American Geophysical Union