Deep seismic profiles recorded in the seas around the British Isles have revealed bands of seismic reflections in the uppermost mantle. On the basis of their appearance, geophysical characteristics, and association with major crustal faults, these features are best interpreted as mantle shear zones. Furthermore, in the North Sea, the mantle reflectors appear to follow an extensional trend and, when interpreted as extensional shear zones, help explain the regional straigraphy of the overlying basin. The crosscutting relationships between mantle reflections to the west of Scotland and a major strike-slip fault, the Great Glen Fault (GGF), suggest that the mantle structure postdates the GGF, and hence is probabily related to subsequent extension. Furthermore, kinematic analysis of lithospheric deformation to the north of Scotland is consistent with the structure observed on the seismic data if the Flannan mantle reflection is an extensional shear zone. Reconstruction of the lithosphere prior to Permian extension restores the Flannan to the edge of the Rockall Trough, a rift probably initiated in the Carboniferous. Thus the Flannan Fault may have developed during Carboniferous rifting. It is concluded that most of the mantle reflections are some form of extensional shear zone in the mantle, and the role of such structures during lithospheric extension is considered. ¿ American Geophysical Union 1993