Detailed pollen analyses, together with analyses of deteriorated pollen and spores and lithological investigations, were carried out at four sites along an east-west transect of the mountains of north Wales. A total of twelve radiocarbon dates were obtained from critical horizons at the four sites investigated. Two of the pollen profiles (Clogwyngarreg in the west and Llyn Goddionduon in the east) provide a detailed record of vegetational and environmental change in the lowlands of north Wales during the Lateglacial (Late-Devensian lateglacial) and early Flandrian periods, while two other profiles (Cwm Cywion and Llyn Llydaw) provide a detailed record of early and mid Flandrian (postglacial) vegetational and environmental history in the uplands of north Wales. Radiocarbon dates from the base of the Lateglacial profiles suggest that the Late-Devensian ice-sheet had disappeared from the lowlands of north Wales by £ 13,500 B.P., and that by that time open habitat herbaceous taxa, including abundant Rumex sp. were colonizing the recently deglaciated ground. The initial pioneer grassland communities were gradually invaded by juniper and birch, but the development of birch scrub appears to have been sporadic, and to have been generally more favoured in the eastern district. An interstadial episode (the 'Lateglacial Interstadial') is recognized in the sequence of plant succession that culminated in the establishment of juniper and birch scrub. A decline in Juniperus pollen frequencies and en increase in herb pollen valu9s (mainly grasses) at the western site of Clogwyngarreg, and fluctuations in the curves for Junioerus and Betula at the eastern site of Llyn Goddionduon suggest that environmental conditions in the latter part of the interstadiil may have been less favourable than those of the earlier/mid interstadial period. By £ 11,000 B.P. widespread solifluction and increasingly severe environmental conditions led to the break-up of existing plant communities and the proliferation of open-habitat and disturbed ground taxa. During this Stadial period (the 'Loch Lomond Stadial') glaciers reoccupied the highland cwms of Snowdonia. Climatic amelioration at the beginning of the Postglacial (Flandrian) was charactarized by a rapid expansion in Juniperus at the low altitude sites, while a pioneer grassland phase, in which Rumex species were prominent, predates the arrival of Juniperus at the recently deglaciated high altitude sites. Early Flandrian vegetational developments between the uplands and the lowlands may have been time-transgressive, but the available radiocarbon dates are inconclusive on this point. Local variations in Flandrian woodland development are described and are thought to reflect both environmental differences and compstitirin factors. At one site (Llyn Llydaw) Flandrian woodland history is traced up to the onset of deforestation in late Neolithic/early Bronze age times.