Field and laboratory studies have been used to assess the main factors that determine the fate and behaviour of agricultural herbicides in the Chalk aquifer of southern England. Field studies using isoproturon, chlortoluron and atrazine showed that leaching of pesticides from normal agricultural usage produces concentrations in Chalk groundwater of 0.01–1 μg l−1 for most compounds, which are comparable with the current EU drinking water standard of 0.1 μg l−1. Where significantly higher concentrations are found in groundwater (up to three or four orders of magnitude higher), these are associated with localized ‘point’ usage or disposal, often combined with more rapid preferential transport pathways to the water table. Studies of the degradation of isoproturon, mecoprop and atrazine showed that these compounds are significantly more persistent in the Chalk than in soils, with half-lives measured in hundreds rather than tens of days. The observed order of magnitude variation in isoproturon half-lives spatially and with time makes the choice of values for predictive modelling problematic. The implications of the results for the quality at groundwater supply sources are discussed and compared with the situation for nitrate, and uncertainties in knowledge, and therefore the requirements for further work on both transport pathways and pesticide persistence, are reviewed.