Throughout 1997 the 28 water companies of England and Wales prepared reassessments of the reliable yields of their groundwater sources using the United Kingdom Water Industry Research (UKWIR) methodology, as required by the Environment Agency (the Agency). An audit of these reassessments was carried out by the Halcrow Group Ltd. The work involved the strategic audit of approximately 5% of the total number of groundwater sources. The purpose of this strategic audit was to ensure that the approach of the water companies to the yield calculations was consistent and followed the UKWIR methodology; quality assurance systems were in place; and all assumptions or deviations from the methodology were justified, and where necessary, were reported to the Agency. The overall level of water company compliance was good. There were several technical issues regarding the methodology, which meant that in places it was not fully followed. General audit observations, implementation issues and technical issues are discussed, and potential improvements to the methodology are suggested.