Forward modeling based on combined geologic surface data, borehole and laboratory data on velocities, and seismic reflection data aid in understanding the seismic response in a transect through complex Alpine nappe structures. The synthetic seismic response from the model compares well with the observed data. The results suggest, however, that strong three-dimensional effects result in out-of-plane reflections and diffractions, as well as defocusing and scattering of seismic rays, thus rendering the use of two-dimensional migration techniques questionable. The model shows an unexpected rapid change in internal structure of the Helvetic nappes between the seismic line and outcrops located 6--8 km farther west. Moreover, the basement-cover interface on the northern flank of the Aar Massif, a basement uplift with a heave of 8 km, does not rise regularly in the hanging wall of a single major thrust fault, but rises in a series of steps due to several thrusts and folds. The northernmost structures appear to be fault-related open folds, whereas tight fold structures appear to dominate in the south. ¿ American Geophysical Union 1993