We use SS-S differential travel times to map the upper mantle structure of the Tibetan plateau and its surroundings. New observations from analog stations in the former Soviet Union help to fill in gaps in the spatial coverage of previous studies, providing new data for eastern Tibet, the Kunlun range, and the Tarim basin. The effects of crustal thickness variations, anisotropy, and global heterogeneity are evaluated and explicitly accounted for by incorporating results of previous studies and by ray tracing through a global 3D model. The resulting residuals suggest that, with the exception of the eastern part of the plateau, Tibet is underlain almost entirely by lithosphere of normal to greater than normal thickness. In general, there appear to be strong lateral gradients in wave speed between eastern and western Tibet. We find little evidence for low wave speeds under northern Tibet to the west of about 87¿E. Instead, the average shear wave speeds in the upper 300 km of the mantle beneath this part of Tibet are about 0.5 km/s faster than those directly to the north beneath the Tarim basin. These results support models of Tibetan dynamics in which the lithosphere underlying northern Tibet underplates the crust as opposed to those in which large portions of this lithosphere disappear by convection, subduction, or detachment. At the same time, the dominant E-W gradient in residuals implies that 2D models inadequately explain the dynamics of all of Tibet.