Chalk is the most important source of fresh water in North-West Europe: more than eight million cubic metres of water are pumped daily from the aquifier. This work presents an authoritative survey of the origin, sedimentology, and hydraulic properties of this unique rock, and the chemical characteristics of the water it contains. The factors influencing the flow through the chalk and its role as a hydrocarbon reservoir in the North Sea are reviewed along with the threats that industrial chemicals, fertilizers, and pesticides present to the supply. The long-term implications of pollutants in this vital resource and the continuing ignorance of the factors that control the flow of water through the rock are highlighted.