The available studies on upper-mantle structure in North America can be broadly divided into two categories: delineation of one-dimensional models, that is, the determination of P- and S-velocities as a function of depth; and computation of two- and three-dimensional models to take into account lateral heterogeneities in structure. About 50 one-dimensional models based on traveltimes, synthetic seismograms, and surface-wave velocities are currently available for continental North America. The gross features of these models are sharp velocity increases at depths near 400 and 650 km in the upper mantle beneath the whole continent and the presence of a low-velocity layer in...