The attention currently being focused on diffuse pollution has led to an increased recognition of the role of groundwater as a pathway for transporting diffuse pollution to surface waters. Understanding and managing this issue requires a good understanding of the spatial and temporal distribution of diffuse pollution in groundwater. This in turn requires a co-ordinated approach to groundwater monitoring, to obtain accurate and representative data. This paper describes a strategic framework for groundwater quality monitoring that has recently been developed by the Environment Agency of England and Wales. The framework is implemented in three main stages: assessment of drivers and objectives, planning and implementation. The national framework is currently being implemented by a series of detailed regional programmes. These are developed on the basis of catchment-scale groundwater quality monitoring units, which will be broadly consistent with groundwater bodies defined under the European Union (EU) Water Framework Directive. The monitoring network that results from the application of the framework is designed to provide background information on groundwater quality at a catchment scale. Separate monitoring programmes will be required for monitoring pollution associated with individual sites. This paper focuses on the strategic aspects of designing a monitoring network, rather than the technical details of obtaining representative and accurate data from individual sample points, and is illustrated by a case study of the process of design of such a catchment-scale monitoring system.