The techniques of modeling upper mantle structure by matching long-period waveforms ith synthetic seismograms have been applied to observations from the tectonically stable part of North America and from Europe. The consistent differences, which can be resolved by the long-period data between Europe and North America, can be interpreted in terms of variations in the crust, lid, and low-velocity zone. At epicentral ranges less than 15¿ the effects of shallow lateral variations are strong and body wave propagation is regionally dependent. Between ranges of 15¿ and 20¿, regional effects are still observed, but they can be explained in terms of variations above 250 km. Beyond 20¿, wave propagation appears to be stable and independent of region. Most of the observed long-period P waves from 20¿ to 30¿ are consistent with a single model. This indicates that the relative depths and sizes of the major discontinuities do not vary substantially. A comparison of the upper mantle models of this study with those of other studies indicates that the shape of the P velocity profile is fairly uniform through the transition region (300--700 km) though there may be differences in the absolute depth to the discontinuities. These differences could, however, be the result of systematic errors in travel time data.