The origin of the East African Plateau and rift valleys is poorly understood largely because little is known about the crust and upper mantle beneath East Africa. The center of the plateau is composed of the Archean Tanzania Craton and is flanked by the Proterozoic Kibaran, Ubendian and Mozambique Belts to west, southwest, and east, respectively. Cenozoic faults of the East African rift system lie primarily within the mobile belts. New estimates of Moho depths, mean shear velocity (V¿s), and Poisson's ratio for the crust of the East African Plateau are presented and used to address questions related to the tectonic development of the plateau and rift valleys. The new constraints on crustal structure are obtained by separately modeling receiver functions and Rayleigh wave phase velocities from teleseismic earthquakes recorded by a deployment of 20 broadband seismic stations spread across Tanzania in 1994 and 1995 and then by combining the results of the separate analyses to obtain estimates of mean crustal structure that satisfy both sets of observations. For the Tanzania Craton, V¿s is 3.79 km/s, Moho depths lie between 37 and 42 km, and estimates of Poisson's ratio are between 0.24 and 0.26. For the Mozambique Belt, V¿s is 3.74 km/s, Moho depths range between 36 and 39 km, and estimates of Poisson's ratio are between 0.24 and 0.27. Parameter uncertainties are ¿0.10 km/s for V¿s, ¿4 km for Moho depth, and ¿0.02 for Poisson's ratio. Results from stations in the Ubendian Belt indicate a V¿s of ~3.74 km/s and Moho depths between 40 and 45 km. Based on a comparison of these results to global averages for Precambrian crust, it can be concluded that there are no regional scale anomalies in the crustal structure that can easily explain the isostatic uplift of the East African Plateau and that Archean and Proterozoic crust in East Africa may be slightly more felsic than Precambrian crust elsewhere. In addition, patterns of crustal thinning beneath rifted areas in East Africa appear to be consistent with amounts of extension deduced from surface structures.¿ 1997 American Geophysical Union