The Porcupine Seabight Basin is a north-south trending, extensional sedimentary basin situated on the continental shelf west of Ireland. It is filled with sedimentary rocks which range in age from Devonian to present-day. A large amount of geological and geophysical data is now available for this basin, mainly as a result of oil industry exploration. In this paper, information from 23 well logs and from regional seismic reflection surveys is used to determine the lithospheric stretching history of the basin. Subsidence analyses indicate that there was one main phase of stretching which began in late Liassic times (~180 Ma), ending in earliest Cretaceous (~145 Ma). Stretching factors at the northern end of the basin are relatively small (β=1.1--1.7) but increase rapidly southwards along the axis of the Main Porcupine Basin. In the southern part of the region (the Seabight Basin), β is greater than 6. The simplest way to accommodate this rapid variation in stretching is by clockwise rotation of the Porcupine Ridge through approximately 20¿, away from the Irish Shelf. Subsidence-derived stretching values are in reasonable agreement with those determined previously from crustal thicknesses based on gravity models and deep seismic data (normal incidence and wide-angle). The existence, location, and inferred age of the Porcupine Median Volcanic Ridge are also consistent with the subsidence-derived stretching factors. After intermittent Paleocene igneous activity, an anomalous increase in the rate of subsidence occurred in the Eocene, between 55 and 42 Ma. There is little evidence that this rapid increase is caused by lithospheric stretching, and we conclude that it must be associated in some unknown way with melt generated by development of the Iceland hotspot. ¿ American Geophysical Union 1993