This document is part of a series of guidebooks that address various aspects of monitoring and assessment of freshwater. It describes the main features of groundwater that govern its quantity, availability and chemical quality. Understanding these features assists in the development of a monitoring programme that will provide information for management and sustainable use. Important principles for monitoring are explained together with methods for obtaining and interpreting water quality data from boreholes and wells. Sources and pathways of contamination in groundwater are discussed, together with approaches to assessing their impact on the quality of the resource.