Extensive fieldwork, laboratory analyses and secondary research on the glacial and coastal phenomena in Lewis and Harris have cast some light on the effects which glaciations had on the development of the coastline. Contrary to previous ideas, Harris and the major part of Lewis were probably covered by a large local ice cap, while the northern tip of Lewis and the Tolsta Head Peninsula were overridden by external ice during the last glacial phase. According to a C-14 date, this phase occurred after about 27,300 B.P. There are indications that a larger part of north Lewis was occupied by external ice during a previous glacial phase. The coastline developed in close correspondence to the pattern of glaciations. The oldest recognisable marine element is represented by a raised shore platform and cliff with an average cliff-base height of 8 m. above the modern cliff base. Because the raised shore platform is widely developed in resistant Lewisian gneiss and overlain by glacial and non-glacial sediments, the episode of abrasion at the higher level probably pre-dates the Devensian. Beach gravel which tops the raised shore platform in places is believed, on the basis of its well-rounded nature and stratigraphic position between two glacial tills, to be of Ipswichian age. The absence of the pre-Devensian rock platform and cliff and beach gravel in central Lewis, south Lewis and Harris may reflect destruction under subsequent ice advance. In this greater part of the island, differential glacial activity of generally higher intensity produced distinct coastal landforms and sediments. There is no unambiguous evidence that the sea around the island rose above its present level during the late Devensian and/or Flandrian. On the other hand, numerous Occurrences of peat below high water mark and one incidence of cemented sand below low water mark point to recent submergence. A dated peat sample form -3 m. O.D. at Holm indicates that, at around 8,800 B.P., the land/sea interface was at least 5 m. lower than at present. The indirect effects of global glaciations on eustatic sea level are considered as having been of greatest consequence to the long-term development of the coastline of Lewis and Harris since the Pleistocene.