Regional seismic reflection studies in the New England and southern Appalachians by COCORP and in Québec by the Ministére des Richesses Naturelles have provided critical subsurface geological information. The data clearly show considerable horizontal transport of off-shelf metasediments over coeval, relatively undeformed, lower Paleozoic shelf and miogeoclinal rocks. In the southern Appalachians, long distance (>200 km) transport of thin crystalline thrust sheets can be shown as well. The COCORP data from the Green Mountains of Vermont and a USGS seismic study in the Grandfather Mountain window of North Carolina and Tennessee appear to indicate that Precambrian (ca. 1.0 b.y.) Grenville basement in those areas is allochthonous and underlain either by shelf sediments or detachment horizons. In Québec, allochthonous basinal facies clastics are still preserved over a major anticlinorial structure, and extensive exposures of Precambrian basement are not found in an internal position in this part of the Appalachian mountain belt. The Bouguer gravity gradient in the central and southern Appalachians and the gravity high in New England and Québec are interpreted to mark a fundamental crustal density change at depth along the mountain chain, perhaps representing a preserved transition from continent to ocean. We infer, in part from the distribution of surface rock units with respect to the locus of the gravity gradient, that allochthonous off-shelf rocks may have been transported farther in the southern than in the northern Appalachians. Perhaps this is true for allochthonous Grenville basement as well, although the question cannot be unequivocally answered at this time. The seismic data suggest that highly deformed rocks exposed in the Appalachian chain are part of a relatively thin, composite allochthon presently confined to high structural levels and that the deeper part of the crust may constitute a largely undeformed ancient continental margin, perhaps including a transition from continental to rift stage or oceanic crust. Surface geologic relationships similar to those described for the Appalachians exist in a number of other mountain belts, and a modern analog for the subsurface structure of the frontal part of the Appalachians is present in the Banda arc of Indonesia. Regional deep crustal seismic surveys are clearly needed in other ancient deformed mountain belts and their active modern analogs.