The Water Framework Directive (WFD) has been the catalyst for a considerable amount of data collection, analysis and research, covering a wide range of physical and biological characteristics and involving a wide range of scientific, social and management-related disciplines. This paper starts with a short description of the discipline of hydrology and then identifies the main hydro logical aspects of work for the WFD, noting their ecological significance. It also provides some idea of the range of organisations and disciplines involved in the work and of the links between them. Of particular significance is the degree of cooperation between river basin districts in setting up working groups for special studies, helping to avoid overlaps in research.