The subsurface geology of the Gulf of St. Lawrence and of the continental shelf off Nova Scotia and Newfoundland is interpreted primarily from the results of 37 seismic-refraction profiles. The Gulf of St. Lawrence is underlain by more than 7 km of Carboniferous sediments deposited in an east–west-trending trough. Another important Carboniferous section about 5 km thick is observed northeast of Cape Breton Island in the Laurentian Channel. The Carboniferous trough north of Cape Breton Island swings irregularly east and south around a basement ridge and may hook into the deep east–west-trending Carboniferous trough extending out of Chedabucto Bay. The Gulf sedimentary basin is separated from the trough north of Cape Breton Island by a sharp rise in basement across Cabot Strait.Northeast of Newfoundland the pre-Upper Pennsylvanian basement plunges to deeper than 6 km in the area of the continental slope. A northeast–southwest-trending seaward-opening sediment-filled trough more than 5 km deep is revealed by the configuration of this basement northeast of the Avalon Peninsula.Seismic-refraction profiles across the Gulf of St. Lawrence between western Newfoundland and Quebec show no indication of rifting or the rotation of Newfoundland away from Quebec. Refraction profiles across the northeast rim of the Laurentian Channel indicate that the rim is erosional in origin and that the basement horizons pass continuously beneath.The seaward extension of the zone affected by the Acadian orogeny is characterized by highly metamorphosed basement with higher seismic velocities, 6.0–6.6 km/s, than the surrounding areas. Limited data northeast of Newfoundland indicate that these high velocities decrease gradually and suggest that the influence of the Acadian orogeny dies away about half way across the shelf.The seaward extension of the Taconic orogenic zone is characterized by an intermediate crustal layer, 6.6–7.6 km/s, at depths of 3–12 km. This intermediate layer continues intact all the way to the continental slope northeast of Newfoundland. It was not observed on the Labrador shelf, suggesting that the Appalachians continued straight rather than remaining marginal to North America.It is postulated that the Devonian (Acadian) orogenic belt was not continuous with a corresponding European orogen, but that the Ordovician (Taconic) orogeny may have persisted across into Europe. If this is correct, it implies the existence of a major rift at the Newfoundland slope during middle and late Paleozoic times.