The Porcupine basin is a north-south-aligned basin offshore western Ireland that contains more than 8 km of post-Paleozoic sediments. The Mesozoic and Cenozoic structure and stratigraphy of the basin have been established on the basis of a large amount of industry seismic-reflection data and 25 exploration wells. Interpretation of the Paleozoic framework is based on some well information and seismic data from the margins of the basin. A series of seismic sections, principally from the western flank of the basin, is used to illustrate the structural styles of the prerift, synrift, and postrift phases of basin development. These structural styles can be related to episodes in the opening of the North Atlantic. A recently acquired BIRPS deep-crustal profile across the ba in extends the conventional seismic record in which reflections are seen below the basin to about 10 sec. At the eastern end of the line, beneath unextended basement, the reflection Moho is at nearly 12 sec, whereas beneath the Porcupine Ridge the reflection Moho is at about 10 sec.