Geophysical and geologic observations in Baffin Bay indicate that the origin of the bay may be ascribed to sea-floor spreading. The bay lies between Greenland and Canada, bounded on the north by the Sounds of the Canadian Arctic Archipelago and on the south by the sill of Davis Strait which separates it from the Labrador Sea. Seismic refraction observations show that the deeper waters of the bay are underlain by oceanic crust. The continent-ocean boundary can be delineated by gravity and magnetic anomalies. The magnetic anomaly at the transition can be explained as an edge effect. The Sounds of the Arctic Archipelago are in general faulted sedimentary basins. Tertiary basalts, known on land east and west of Davis Strait, also are extensive offshore from western Greenland, and in some places cover older sediments. These basalts have characteristics similar to those which some workers believe to be typical of products of hot spots. The opening of the bay can be represented geometrically by the combination of two motions, a rotation about a pole in the Arctic Archipelago, and a translation along Nares Strait. Indirect evidence from the Labrador Sea, the Innuitian province of the Archipelago, and the Tertiary volcanism suggests that the bay attained its present configuration during the Late Cretaceous and Tertiary, but evidence for timing from the bay itself is poor. The timing suggested does however agree with the idea that sediments eroded from the Innuitian province during the Cretaceous and Tertiary were channeled through a fluvial system which occupied t e sounds, and were deposited in northern Baffin Bay where the present gradient of the continental slope is gentle.