This Report is revised and expanded from the 1972 publication, providing an up-to-the-minute account of the British Ordovician formations and their correlation nationally and internationally. It also includes the most comprehensive treatment of Ireland ever attempted. The reference list is a comprehensive bibliography of papers on the subject published since 1970.This Special Report will be a valuable reference for research and applied geoscientists working with rocks of Ordovician age. It will be of particular interest to those working in, or visiting, the Welsh mountains and the English Lake District.