In the polymetamorphic areas of Scotland and Ireland, rocks that yield isotopic ages of about 420 m.y. are widespread. The Moine rocks of western Scotland range from 370 to 570 m.y. and some may be as old as 740 m.y. The Dalradian rocks range in age from about 420 m.y. to about 500 m.y. Direct geologic evidence bearing on the age of the main metamorphic events is limited to the presence of detrital Dalradian metamorphic material in the Arenig rocks of Connemara in Ireland, and the involvement of fossiliferous Lower Cambrian rocks in the upper part of the Dalradian of Scotland; there is some suggestion that the Cambrian-Ordovician rocks of the Moine thrust zone were involved in early movements that affected the Moine nappe. It is not possible yet to reconcile completely th isotopic ages and the geologic information. Controversy about the interpretation of the age pattern centers on the importance of overprinting of early metamorphism by later metamorphic events and on the effects of late orogenic cooling. Correlation of events in the areas of high-grade metamorphic rocks with stratigraphically documented events in nonmetamorphic areas of the British Isles is an unattained goal. The overstep of the Old Red Sandstone on older rocks is the result of an accumulation of movements during the Ordovician and Silurian. It can be argued that the 420-m.y.-age peak found throughout a large area of the Highlands is merely a minimum (i.e., cooling age) for a middle Ordovician event or, more probably, a close approximation of the true age of a Silurian event. N merous isotopic ages of igneous rocks intruded in late Silurian to Early Devonian time show that the main period of granite emplacement took place 400 m.y. ago, and that the Moine thrust movements took place at about the same time. The first movement along the Great Glen wrench fault occurred after the intrusion of the Foyers-Strontian granite (400 m.y.) and before the deposition of the local Middle Old Red Sandstone. The Hercynian orogeny in England occurred about 295 m.y. ago.